Also, once no air is coming out of a bleeder screw, close it finger-tight, then wait for the others to clear up. When they're all closed, press the brake pedal a couple of times just an inch or two by hand. That will wash the last few remaining air bubbles into the calipers and wheel cylinders. Open each bleeder screw once more for a couple of seconds so those bubbles can spit out.
If you're just exchanging the fluid to get the old moisture-laden stuff out, like every manufacturer recommends but no one does, there won't be any air in the system as long as you watch the master cylinder so it doesn't run empty. If you are replacing a caliper or wheel cylinder, the brake fluid will be dripping out while a hose or steel line is disconnected due to, ... Uhm, ... Gravity, and the reservoir will run empty creating the need to bleed the system. A simple trick to avoid that is to place a stick between the seat and brake pedal to hold the pedal down two inches. Unless gravity is uncommonly strong where you are, it won't be strong enough to pull the brake fluid past the lip seals in the master cylinder. Loosen the reservoir cap like djcl said, otherwise vacuum will build up as the fluid runs out and that will prevent more fluid from running down. It's best to leave the cap loosely in place to reduce the amount of moisture the fluid sucks out of the air. Brake fluid loves moisture but that will promote corrosion of metal parts and it greatly reduces the fluid's boiling point leading to brake fade and a mushy brake pedal. Always keep your can of new brake fluid tightly capped accept when you're pouring from it.
There's one more important thing to be aware of that only applies to GM front-wheel-drive cars. When pedal-bleeding with a helper, or after replacing calipers or pads, never, never, ever press the brake pedal more than half-way to the floor. That is proper procedure for any car or truck when the master cylinder is more than about a year old because corrosion builds up in the lower halves of the two bores where the pistons don't normally travel. Pressing the pedal to the floor runs the lip seals over that crud and can rip them causing a slowly sinking pedal and eventual failure of the master cylinder. But there's a more important reason on GM fwd cars. Most fwd cars use a "split-diagonal" brake hydraulic system. Instead of a front and a rear system like we've had since the 1960s, the left front and right rear are on the same hydraulic circuit. GM master cylinders have a valve that trips to block off two ports when one circuit doesn't build the same pressure as the other one. That happens when one circuit has a leak but it will also occur when you stroke the pedal to push the pistons out of the caliper housings. One piston will naturally come out first and when the pads contact the rotor, that side will start to build pressure. Since the other piston hasn't moved as far yet, no pressure will build and that valve will trip. From that point on, no matter how hard you press the brake pedal, no fluid will come out of that caliper and the opposite wheel cylinder. The service manual says that can be avoided by pedal-bleeding the wheel in a specific sequence, but they lied. That valve will trip if ANY wheel builds pressure while ANY wheel does not.
Once that happens, many people identify the master cylinder as the reason they can't get brake fluid from two wheels and they replace it, then run into the same problem when bleeding the new one. This can happen even when not bleeding anything. Just pumping the pistons out after replacing the front pads can cause this problem. The only way I have ever found to reset that valve is go to one of the wheels that isn't flowing fluid and give a very short, quick burst of compressed air through the opened bleeder screw, then let it gravity-bleed.
Some owners don't even notice when that valve has tripped. Suspension geometry and alignment angles have been modified on front-wheel-drive cars so there will be no brake pull when one side isn't applying. On Chryslers you'll never see the slightest hint of a pull. On most GMs, all you'll see is a slight wiggle of the steering wheel as the brake pedal is being applied. A fairly common complaint on GMs is those new front brake pads wore out too quickly on one side and the other side looks like brand new. That's because the one side is never applying and one side does all the stopping. Some people don't ask for help until they've gone through two or three pairs of pads, then they figure out something is wrong. All this can be avoided by never pushing the brake pedal more than half-way down to the floor.
Thursday, May 17th, 2012 AT 5:28 PM