I serviced my brakes (both front & back) about 18 months ago. Had all 4 rotors cut, & installed new ceramic pads & replaced emergency brake hardware as well. Vehicle drove fine & I have only put about 4K miles since that time. Recently I started to develope pulsation in pedal & a very choppy feeling when braking. Inspected all 4 wheels. Pads look fine, pull rotors off & checked for warpage by laying metal straight edge across rotors. They look fine to me & rotor surfaces show normal wear. Caliper pistons retracted back-in easily using a C-clamp. Also I checked to make sure that I don't have any play in the front wheels from top to bottom. All seems tight with front end. While the front calipers were detached from rotors with pads off, I had someone lightly pump pedal with engine running & I noticed that the top piston (front calipers have dual pistons) pertrudes outward while bottom piston barely moves. This is true for both sides.
Is this normal for the top piston to pertrude outward (after 4 or 5 taps of the brake pedal) while bottom piston barely moves & if so why?
Next as I was reading over your info on replacing rotors & caliper; you mention the caliper should move freely from side to side on the slide rails.
How is this checked? Is it done with the pads & calipers re-attached & before touching brake pedal (while pistons are still retracted)? Also how much play should there be?
Lastly, any thoughts on what is causing this pulsation & choppiness when applying my brakes? Chopiness is noticeable to anyone riding in vehicle.
Please advise back @ earliest convenience. I will increase my donation should the amount of written exchanges warrant it.
First of all, a straightedge is not nearly accurate enough to check for warpage. That might tell you if either plate is not flat but the rotor could have thickness variation and the friction surfaces could be perfect but sitting on a non-parallel center mounting surface. You must either mount them on a lathe or you can use a dial indicator to check for sideways runout.
As a general rule, thickness variation will only make the brake pedal pulsate, but lateral runout will tug the steering linkage and wheel back and forth without affecting the brake pedal. You will have to use a micrometer to measure thickness variation by measuring at six to eight places around the rotor. Look for the thinnest and thickest points to calculate the maximum difference. A total difference of.002" -.004" is more than enough to feel in the pedal. That is the thickness of a sheet of paper. You 'll never see that with a straightedge.
Since the pistons move freely you don't have to worry about which piston moves first. Both will never have the exact same resistance to moving so of course only one will move first. Just place a piece of wood in there to block the moving piston, then the other one will move. Each piston has a "square-cut seal" around it to seal the fluid in. That seal sticks to the piston and bends when the piston moves out. When you release the pedal, that seal wants to straighten out. That's what pulls the piston back slightly to prevent the pads from dragging on the rotor.
It's true the calipers should be able to move freely back and forth on their mounts but Ford has always been known for their very poor designs and sticking calipers. Newer models are finally using mounting bolts like GM and Chrysler have used for decades. Those bolts should be replaced if they are bent or have any rust pits and lifted chrome. They should also be lightly lubricated with high-temperature brake grease. Older Ford trucks used steel wedges with rubber inserts If you still have that design, you will need a sledge hammer to move the caliper. (That is not sarcasm).
February, 21, 2011 AT 6:10 PM
It was extremely hard 2 locate my answer. Anyway I don't understand what is meant by your response about "Older Ford trucks used steel wedges with rubber inserts"? When U say older Ford trucks does that apply to my 2001 F-150 truck? Finally, other than your instructions on how to check my rotors for warpage (thank U for that info as I will check them) U didn't offer any other info to me as to what U thought the likely problem is with my brakes. Do can please tell me what else could be the problem. Please advise back. Thank You. Bob
February, 22, 2011 AT 1:30 AM
Warped rotors. There is no such thing as checking them with a straightedge. I'm not the best at describing the different types of warpage but I'll give it my best shot. Imagine if you had the rotor mounted on a brake lathe. The cones that hold it grasp it by the mounting plate right around the stud holes. That is the surface that it must be indexed on because that is what will sit flat against the hub on the truck. Now, two cutting bits, one on each side, remove the high spots and make each braking surface perfectly parallel to that mounting surface. Along with that, the thickness between those two braking surfaces is exactly equal all the way around.
The first type of warpage is thickness variation. That must be measured in six to eight places around the rotor with a micrometer. As the rotor turns, when that thicker spot goes between the brake pads, it pushes the pistons back into the calipers. That pushes brake fluid back up to the master cylinder and pushes the brake pedal up against your foot pressure. When the thinner part of the rotor goes between the pads, the pistons can come out, brake fluid takes up the space behind the pistons, and the brake pedal goes back down a little. .003" thickness variation is plenty to feel in the brake pedal. That is the thickness of a sheet of paper.
The second type of warpage is a whole lot easier to describe with a group of students standing around the lathe so I can point to things. Because you have a 4wd truck, you have rotors that slide on over the studs. Imagine it is mounted on the lathe but there is a piece of rust or scale stuck between one of the mounting cones and the mounting surface of the rotor. The entire rotor will wobble as it rotates. Now you machine both sides. When you're done both sides will be perfectly parallel to each other, there will be absolutely no thickness variation, but the entire braking surfaces are not parallel to the mounting surface. When you mount the rotor on the nice true hub on the truck, the braking surface will have "lateral runout". If it's bad enough to see, you will see the brake caliper moving left and right as the rotor goes around. There is no thickness variation so you won't feel anything in the brake pedal. All brake rotors develop a little lateral runout but if the caliper is free to slide left and right, it is unlikely you will feel it. I looked up your caliper mounting system, (here's a link to what you should have):
Even that style of caliper retainer is relatively forgiving because there is rubber sandwiched between the two metal parts. That rubber lets the caliper move back and forth a little with the lateral runout in the rotor.
To add to the confusion, there are two different times warpage can appear in a rotor. Most people assume it can only happen on the truck from them becoming hot, but it can also be machined in accidentally on the lathe. I already mentioned the chip of rust or scale getting stuck behind the mounting plate. That will most definitely put lateral runout in the rotor. A much less known cause is over-tightening the nut on the end of the shaft. The mounting cones that hold the rotor are never perfectly true but that doesn't usually cause a problem if the nut is just snugged up. Some mechanics think that nut must be tightened as much as possible, but that will bend the shaft of the lathe by forcing the cones tighter together. That is very easy to see when the lathe is spinning. The end of the shaft will appear to move up and down as it rotates. That WILL result in lateral runout.
If the caliper is free to slide on the mounting sleeves, you will likely not feel that lateral runout, at least at first. Eventually any brake grease will wear off, and dirt and water will get in there and reduce the caliper's ability to slide freely. If the caliper refuses to slide, it will force the rotor to move sideways a little to center itself between the brake pads. Tugging on the rotor means it's also tugging on the hub the rotor is mounted to. That tries to turn the wheel and is why you will feel that tugging in the steering wheel.
There is a third thing you must look for on your rotors, if this applies. Rotors that slide onto the hub, such as yours, often have an access hole in the hub itself. There are the five, (or six) holes for the wheel studs, but there is often an additional hole that allows you to reach the bearing assembly mounting bolts with a socket and extension. Water will spray up in there and form a small circle of rust on the back side of the rotor. This is extremely common on all brands of vehicles. That rust spot, (sometimes three of them), must be scraped off before the rotor is machined. The mounting cone will rest on that rust and cause the rotor to sit crooked on the lathe. That will result in lateral runout being machined into it. Even if the cone doesn't rest on that rust, the rotor can be reinstalled five, (or six) different ways. If that rust spot doesn't line up with the hole that let it form, it will be wedged between the hub and rotor's mounting plate. You might not notice that right away on a heavy truck but you WILL feel it on a light weight car. This was a real common problem on GM cars. It wasn't as noticeable on Chrysler cars because they had three holes and three spots of rust buildup so it was more likely the rotor would still mount parallel to the hub.
The only way to measure lateral runout is with a dial indicator as shown in this link:
The stem near the lower left side is placed perpendicular to the braking surface and partially pushed in, then you watch how much the pointer moves as the rotor spins. This model can measure a total of one inch. That is ten revolutions of the pointer. There's 100 graduations around the outside. Each one represents .001". It is common to find .003" to .005" runout in any rotor. By the time you get to .010" you will probably start to feel it in a small car but not necessarily yet in a truck.
When doing this on your truck the lug nuts must be installed to hold the rotor in place the same as it will be when the wheel is installed. We don't normally have the time to do this but if a mechanic is really conscientious, he will measure lateral runout this way before he removes the rotor for machining, then he will measure it again when it is mounted on the lathe. You know the hub on the truck is true so if the rotor is mounted properly on the lathe, you should measure the same amount of runout. That extra step is usually skipped because it's so seldom we run into a problem. It's the one in a hundred that give us trouble where we need to resort to this type of test.
After performing a brake job on a mid '90s Dakota many years ago, I accidentally machined in severe lateral runout. That one used the same type of slide-on rotor that you have. The only hint of a problem on the test drive was a clicking sound once per tire revolution when the brakes were applied. After lots of frustration, it turned out to be one inner pad sliding sideways along with the caliper and catching on a notch in the mounting knuckle. That notch was the result of rust and wear over many miles. While running it in gear and jacked up, you could easily see the caliper moving sideways about 1/16" as it tried to stay centered over the rotor. There was no unusual feeling in the brake pedal or steering wheel. Had I ignored the noise, it would have eventually gone away when the lining wore down and the metal backing plate became centered in that fairly wide notch.
As far as your question about checking the calipers' ability to slide, that shouldn't even be an issue with your truck. "Newer" to me means anything newer than a late 1980s model which is what I drive. In the '70s Ford used steel inserts with a steel spring-metal wedge to hold the calipers. You actually needed a large hammer to slide the caliper sideways. The idea was that as the outer pad wore down, the caliper would eventually slide from heavy pedal pressure. Since it would never slide back, the outer pads wore very quickly if you always used the brakes hard. If you were light on the brakes, only the inner pads would apply and wear faster. That was a very poor design. When I worked at a Sears Auto Center in the '80s we saw a lot of those trucks come in with grinding brakes at less than 15,000 miles.
The rubber wedges shown in the second link above is an improvement but why did the engineers think the caliper would slide to take up the pad wear when the rubber in that insert could just bend, and then straighten out when the pedal was released? Uneven pad wear was still a big problem. The best design is what you have on your truck. With the caliper still mounted in place, you should be able to easily pry the pistons into the caliper with a large flat blade screwdriver. If you can not, the piston could be sticking. That's a whole different story for another day. Once both pistons are retracted, you might be able to slide the caliper by hand. Don't panic if you have to bang it with a small hammer. It's sitting on rubber inserts that ride on those metal sleeves so it will stick a little. It will slide over time to accommodate the outer pad wear, and it will slide a lot easier than either of the previous designs.
February, 22, 2011 AT 9:13 AM
Thank you for your last response as it was very informative. A couple of things that I would like to mention & they are as follows; 1) One of the links that you provided me with show caliper mounting bolts & slides. My 2001 F-150 doesn't have what I consider the typical long mounting bolts as shown on that link page, rather my mounting bolts are quite short. They are only about 1/3 of the length shown on that link & of most of the vehicles that I have ever owned & or worked on. And therefore the slides are very short as well. The mounting bolts go right thru the inside mounting hole into the rubber bellows which contains the threaded end to capture the mounting bolts. So I'm wondering if allot of what your telling me is simple "stock" cut & paste text that doesn't even apply to my style vehicle. Am I not talking to a car pro that is a Certified Automotive Technician?
2) As for all the info that you replied to me about on brake rotor surface warpage & how to check for, that is all good stuff & I appreciate that. On that same subject, when I got my rotors back from being cut they indeed still had a small lip of rust around the OUTER edge of the rotors on both the inside & outside surfaces. Also there is a rust ring on the INSIDE surface of the rotor around the hub opening. Though the ring isn't that thick I don't think that it can be knocked off with sand paper. Can it be somehow scraped offed slightly & or grinded off or are you saying that these lips need to be absolutely machined off? If so then I guess from what previouslt stated that the entire rotor surfaces will need to be re-machined (if there is enought wall thichness left) is that correct? If that is the case as U have pointed out then these rotors if previously machined with the rust still intact do undoubtedly have runout. So I'm guessing that I would be better off just picking up a new set of rotors? To that end can U also tell me if you would agree or not that because the majority of the stopping is done by the front brakes & along with the difference in design between the front & rear rotors that it's MOST LIKELY (or not) that the front rotors are my problem child?
3) Lastly, even though I believe I already asked U this & U haven't made any other suggestions back to me; do I take it that there are NOT any other likely causes (based on the original info I supplied to U) that U feel could be causing the pulsation in the brake pedal & this very hard CHOPPINESS in the overall feel of the truck (feels like the bed area is banging up & down (for a lack of a better term) when my brakes are applied when traveling @ let's say over 20 MPH?
That's all 4 me, if U could be so kind to address the 3 above questions/issues 4 me I won't bother U any further. Also, if U could suggest to me what if any additional amount over the $15 donation that I have already made U feel would be appropiate for your additional responses back to me, than please pass your suggestion along to me with your forth coming response to my final questions. Thanks Very Much! Bob
February, 22, 2011 AT 12:41 PM
If a mechanic could have memorized 5000 pages of service manuals in the 1960s he could have repaired every car model in the world. Today 5000 pages doesn't cover all the diagnostic manuals for one model for one year. No one can memorize everything. That's why I use photos to refresh my memory. The first two were from rockauto.com. After looking again, I also found this setup:
Since you can't go back a page from the links that are entered directly, you can do your own searching by going to http://www.rockauto.com, select the year, "Ford", "F-150", engine size, "Brake / Wheel hub", "Caliper Bolt". That way you can "page back" to look at other things.
I do copy and paste sometimes because I often type for over an hour, then get asked the same question a few days later. After describing the same thing over and over, it gets real easy to leave out a few important details. Everything I paste is something I typed earlier. I have hundreds of documents saved in MS Word but I still end up rereading them and making changes as necessary for each person.
As for my sorry credentials, I've been an ASE Master Technician for over 30 years. That means I know how to take a written test. (Sarcasm intended). I was a brake and alignment specialist at a Sears Auto Center and a steering, suspension, and alignment specialist at a very nice family-owned Chrysler dealership for ten years. During those years I was also a certified electronic technician (CET), which is a fancy name for a tv / vcr repairman. After leaving the dealership, I taught in a community college for nine years. My areas were suspension and alignment, brakes, electrical, and engine rebuilding and repair. I've been unemployed for over two years so your donation is greatly appreciated, but I think you've already donated more than most people, and I thank you. We don't get to know how much you donated, and I like it that the site owners set it up so people don't HAVE to donate to get an answer. I type just as long and do just as much research for people who didn't donate, although I'm a little pickier about which questions with donations I try to answer because people like you have more invested and deserve an answer from someone qualified. There are a lot of questions I could give a generic answer to and that would satisfy the typical do-it-yourselfer, but then I risk running into someone such as yourself who needs more in-depth information that I might not be able supply. If a person is kind enough to make a donation, I'm not going to answer unless I'm fairly confident we can resolve the problem. My record is over 35 replies with troubleshooting steps to one person's intermittent electrical problem, and we did finally figure it out.
To get back to your truck, when I clean the rust from inside the rotors I use a right angle air tool with a fiber scuff pad similar to a Scotch Brite pad. That rust ring outside of the hub contact point is not a concern when it goes back on the truck because it will be sitting on the same area as before. It COULD be a problem though if the brake lathe mounting cone contacts that raised area. Most brake experts know to check where the cones will make contact and to clean the rust off if necessary but it's always possible the person running the lathe didn't catch it or thought the cone wouldn't hit it that rust.
Any small rings of rust along the inner and outer edges of the braking surface are nothing to worry about. Sometimes the cutting bits don't reach all the way in far enough without a lot of time-consuming setup adjustments. Usually the pads don't make contact all the way to the inner-most edge of the rotor anyway. Any rust on the outer edge will grind a matching groove on the new linings during the test drive. You won't hear it after that. If the pads do run in that area, the rust will be worn off in short order.
The way you describe the pulsing, I'm more inclined to think there is thickness variation. You can get an identical sensation from a locked up caliper that won't release but that will be accompanied by the truck being hard to move, and that brake will get very hot. The clue can be found by opening the bleeder screw. If the caliper doesn't release, it is due to the caliper piston. If it does release, it is due to trapped brake fluid. The rubber hose can cause that if there is a metal bracket crimped around the middle of it. Rust builds up inside the crimp and squeezes the hose closed. Simply peeling that crimp open a little with a big pliers is all it takes to solve that. That is more of a Chrysler thing; in fact, it happened to one of my Caravans that had been sitting all winter, but it's worth mentioning. The clue is the brake will not release when the steel line is momentarily loosened at the master cylinder. By the way, a locked caliper will cause the severe pulsing you described even when you are not pressing the brake pedal. Thickness variation will only cause the pulsing when you apply the brakes. You'll need a micrometer to measure the thickness in a number of places around each rotor. I think you're going to find between .005" and .010". I doubt this happened from having them machined because first of all, you would have noticed it right away, and second, most lathes have two cutting bits to machine both sides at the same time. You can't help but end up with two perfectly parallel plates. Some really old equipment have just one cutting bit but they will still work fine unless the rotor is removed and remounted between cutting the two sides. It's next to impossible to guarantee it will mount exactly the same way.
On trucks that use the twin I-beam front suspension, a worn strut rod bushing can cause a severe hop when the brakes are applied. It lets the I-beam move forward and backward and amplifies the thickness variation. Your truck should have upper and lower control arms which are less susceptible to that hop but it wouldn't hurt to look at their mounting bushings. If there IS a sloppy one, you will usually see some choppy tire wear too. To inspect them, support the truck by the frame or cross member so the lower control arm is free to move, then reach in and pry on them with a pry bar. The rubber will flex a little but the arm shouldn't move very much.
I looked at a photo of the rear rotors. If you have five lug nuts per wheel, it appears you have a solid, non-vented rotor. Thickness variation is less common with them but check them anyway if you don't find the problem on the front. Also look for the presence of shiny "hot spots". Those are small spots of very hard metal that the cutting bits skip over. As the softer metal wears down those hot spots appear to grow and raise up. If they're bad enough they will cause the same sensation as thickness variation by grabbing the pads.
When you put everything back together be sure to use a click-type torque wrench on the lug nuts. Uneven tightness will lead to lateral runout caused by the uneven clamping forces on the rotor. This will occur after a number of heating and cooling cycles. This is a bigger problem with the hub and rotor units cast as a single piece, but it is still important with your slide-on rotors.
My last comment has to do with grease. If any grease gets on the braking surfaces or the linings be sure to wash it off before they get hot. When the linings get hot the grease will soak in and can cause grabbing and a squeal. I even get nervous about fingerprint grease so I avoid handling rotors by their friction surfaces after they've been machined.
Well, I guess it's time to get a few hours of sleep! Let me know what you find.
February, 23, 2011 AT 2:13 AM
We just got about 6 inches of fresh snow today so I most likely won't be outside playing mechanic for a couple more days. The upper & lower control arms are something that I will have to re-look at. When I looked over the front end previously I didn't notice any worn or busted bushings but I will definetly take a closer look now. The thing U mentioned about the rubber brake having a metal bracket crimped around the hose & the rust build-up, I did notice a metal (not sure if it's a brk't or transistion piece from rubber hose to metal line, but I was picking the rust out (as much as I could) from the inside of that tube. U may have something there as well. Although I haven't smelled a heated rotor or brake pad I can say that all my brake pads are showing even wear. In addition I did notice the other day that when I put my truck in drive it didn't move until I tapped the gas pedal which lead me to initially think that my emerency brakes were possibly set to tight as the inspection idiots in Jersey check every f'n thing or at least they use to. In a move to save money by the state of NJ they now only check emissions every 2 yrs instead of everything humanly imaginable. When I did check my e. Brakes they were dragging the rear wheels a little more than I would like so I did back them off a bit, but I noticed the truck still didn't move at idle speed when put into gear so again you are rasing another good point. I think U have now given me enough suggestions on things that I need to take a close look at. Lastly, I didn't realize that U could page thru the links U sent me I simply just saw the page that loaded & thought "that doesn't apply to me" kind of thing. As 4 your credentials one doesn't know unless one asks! I will let U know what I find in a day or so when I get back out there. Thanks again. Bob
March, 6, 2011 AT 3:43 PM
It's me I'm back. Between being sick & 2 of my kids being sick I finally found my way back out to my truck. I started from scratch so to speak yesterday by going back over everything on both my front & back breaks. This time I pulled everything off both my front & back brakes & here is what I found. On my front passenger side caliper I have one leaky piston boot. I may have caused this myself by over extending the piston (forgot to block it off with piece of wood) while checking out the driver side front caliper. What are your thoughts on using a rebuilt seal kit as opposed to replacing the caliper? Next thing I noticed this time around is that on the front driver side pads the inner pad has a slightly uneven wear/chatter looking mark on the bottom 1/3 of the pad. Though the uneven wear was noticeable I was able to take a piece of fine sandpaper along with a piece of emery cloth and it came right out. On that same side I found a large amount of rust around the inside diameter surface of the rotor where it makes contact with outer diameter of the spindle surface that holds the lug studs as well as around the beveled surface that goes around the large center access hole. This would be what U were referring to I assume. I can see how the rust on the rotor wouldn't allow the rotor to mount up competely parallel to the caliper & pads. Lastly, when I jacked the rear of the truck up & cracked open the bleeder valves on both back calipers, I found that I am NOT getting any fluid out from the driver side caliper & there is a large amount of drag on the back wheels even with the E.B is backed off. I checked the brake line from the caliper back to the "tee" that supplies fliud to the passenger side. The brake hose and steel line looks to be fine. I don't see any pinch or kink in any part of the line. I even took the bleeder valve completely out in case there was a blockage in the valve, but still no signs of brake fluid. Can the caliper have a internal problem that could cause this? I going back out today & I'm going to loosen the brake line where it meets that "tee" I spoke of to see if there is fluid at that point. Also unless I'm going crazy I could have sworn that last time when I had the back of the truck jacked up & I was turning the tires on one side the opposite was turning the opposite direction. This time around it's not. What's up with that? Your expertise on this matter would be greater appreciated. Please advise. Bob
March, 6, 2011 AT 5:10 PM
Extending the piston didn't rip the dust boot. It would have simply slid off the groove in the piston. Ford boots can be particularly difficult to replace because the piston must be completely removed, the groove in the caliper housing must be cleaned extremely well, then the piston must be used as the handle to set the boot in place. Pressing the piston in locks the boot to the caliper. Chryslers work the same way but for some reason I always ran into trouble on Fords with that boot peeking out in one spot so I'd have to start over. There is also a pliers available that allows you to press the boot into the caliper first, then spread the boot so you can drop the piston in. What you might consider instead is using a silicone gasket sealer to patch the hole. I'm very familiar with two Chrysler products. Ford and GM have similar sealers. The gray stuff gets harder and will still bond and seal if there is a light oil film on the parts. I used this to patch two major holes on gas tanks. The black stuff will be a better choice for the boot. It stays more rubbery but the surface must be clean and dry. All you're trying to do is keep dirt and water out. Once the caliper is installed, that boot will only extend once over the life of the pads so the patch won't have to flex very much. There's a 99 percent chance you will not have a problem due to the boot so replacing the calipers is kind of overkill. What would be more likely to happen is dirt gets inside the boot and becomes impacted on the piston or water gets in and causes rust pits if they are chromed steel pistons. The problem will occur the next time the pistons have to be retracted to install new thicker pads. The rust pits and dirt will hang up on the square-cut seal inside the caliper. You know you have a problem when you HAVE to resort to a c-clamp to retract a piston. That dirt will cause the piston to not apply properly under light braking and it will not release properly after heavy braking.
I don't think the rust on the rotors is the problem because eventually it would have crumbled and would have made the lug nuts loose. However, if I failed to mention it earlier, check for access holes in the mounting plate of the rotor. Water can spray up there and form little circles of rust. Normally those get scraped off before the rotors are machined. If they were not, a warp could have been machined in or those spots could hold the rotor from being perfectly parallel to the hub. Come to think of it, I think we covered that already. Freshly-installed calipers might slide freely enough to accommodate that warpage but later the calipers might start to resist sliding enough to cause a pulsing sensation.
When the transmission is locked in park so the drive shaft can't turn, the rear wheels will turn in opposite directions. With a standard differential you can do that by hand. If you have a locking differential you usually won't be able to turn the wheels by hand, depending on the type it is. Very often the two wheels will turn forward together because it is fairly easy to turn the drive shaft that way. It is turning on an over-running one-way clutch in the transmission. That clutch locks up in the reverse direction and forces the drive shaft to spin some rotating members so you will find the wheels turn harder backwards. That will make the other wheel want to spin the opposite way instead. So you're not losing your mind. It has to do with direction and whether it's in park or neutral.
Don't have an answer about the lack of fluid to the rear but you're approaching it the right way by removing the line. You might try having a helper press the brake pedal, but no more than half way to the floor, or you could prop a stick between the seat and brake pedal, then run back and open the left rear bleeder. I've had some wheels that didn't seem to want to gravity-bleed on their own but I never gave it much thought. I just grabbed a helper to work the pedal.