Don't waste your money on a used master cylinder because you don't know its history. Any of a number of things could have damaged it the same way as your old one. Removing front calipers, disconnecting the power booster, or anything else that allowed the pedal to go down too far could damage the seals. If a cover was left loose, moisture would be drawn into the fluid increasing the corrosion and likelihood of seal damage. Sometimes people taking a car to the junkyard purposely damage things. One way to destroy EVERY part in the hydraulic system that has rubber parts is to add just a little petroleum product to the fluid. Evidence of this is the rubber seals under the caps ballooning up so you can't reinstall the caps. Also, if someone removed a part from the hydraulic system, the fluid would have drained out. The dry seals will deteriorate very quickly and will wear out from rubbing on the dry surface in the master cylinder bore.
Also, in quite a few cases, the rebuild kit costs more than the rebuilt unit with a warranty. When you do rebuild a master cylinder, you will notice that these are made from aluminum to lessen vehicle weight. To prevent rapid wear and corrosion, the complete body and bore is protected with an anodized coating, usually light blue. You must absolutely not scratch this coating in the bore. That means no cylinder hones, sandpaper, or picks like we used to use on older cast iron master cylinders in the past. Cleaning chemicals and lint-free rags are the only things allowed in the bore.
Regardless of whether you rebuild your old master cylinder or buy a rebuilt unit, be sure you understand how to bench bleed it before you install it. Most come with the bleeding kit which consists of two short hoses, the two proper fittings, and instructions. Some fittings, (with holes on the sides of the outlet nipple), form check valves that prevent fluid from flowing back in the port when you release the piston. If the holes are on the ends of the fittings, like most are, you will need to push the piston in with a stick or screwdriver, then pinch the hoses when you release the piston, to prevent just moving air back and forth as you stroke the piston.
You will never get rid of all the air during bench bleeding because a little sneaks back in by the threads of the fittings when you release the piston. A little fluid will seep out there too. Not much you can do about that. It's a messy job. In this case, you CAN push the piston all the way in without damaging the seals because there is no corrosion in there yet.
Leave the hoses connected and pinched off as you install the unit on the car. This will reduce dripping. Brake fluid will damage paint so wash any off right away if it gets on the body.
Most Fords and GMs have four steel lines connected to the master cylinder so this trick won't work. There must be only two lines on the same side, like most Chrysler products. Most people just unbolt everything, then pull the old master cylinder out. Fluid dribbles out of the lines, and later, air is pushed down the lines resulting in the need to bleed at all four wheels. Some air can get trapped in a high curve in a steel line and won't completely bleed out. That will cause a brake pedal that is always low and mushy. That's what my method avoids.
To prevent this from happening, loosen the two line nuts just a little, THEN unbolt the master cylinder from the power booster. Pay very close attention to the steel lines as you turn the nuts to be sure they're not twisting. If the nuts don't spin freely, the lines will twist and snap off. Then you have another repair to do that involves new fittings and making double flares. Double flares are hard enough for a professional. They're harder to do in the confines of the engine compartment. Pull it off the mounting bolts, then twist it up so the two lines bend upward just enough so those first two inches are not parallel to the ground. Then you can remove the lines. This will prevent the brake fluid from running out of them. Be careful to not drip brake fluid on the car's paint.
After bench bleeding the new master cylinder, leave the reservoir at least half full of fluid. When you install it, you will have to tilt it to line up the ports with the line nuts. Remove one plastic bench-bleeding fitting, then install the steel line nut hand tight. While you do this, fluid will be dripping out, keeping the line and port full. If the nut doesn't thread in easily by hand, it's cross-threaded. Start over and try again until the nut goes in by hand, typically four or five revolutions. If you damage the threads in the aluminum ports, there will be no warranty and you will have to buy another master cylinder.
Once the first line is connected hand tight, do the same thing with the second line. When both fittings are hand tight, twist the master cylinder and lines back down to their normal position, and bolt it to the booster. You'll need a helper for the next step. There will be a little air in the ports and tops of the lines. Tighten the front nut, then loosen the REAR nut about a quarter turn, then have your helper push the brake pedal down very slowly. It should take them about 15 seconds to push it half way to the floor. Any faster, and the air might still get forced down the lines. While the pedal is moving down, you will see air bubbles coming out around the nut's threads. When the bubbles stop appearing, tighten the nut. Tell your helper to not allow the pedal to move back up until you tell them the nut is tight. Raising the pedal too soon will allow air to be drawn back in through the nut's threads.
When the nut is tight, tell your helper to allow the pedal to come back up QUICKLY. The brake fluid rushing back to the reservoir will wash any air bubbles back too. Do this procedure a second or third time until you don't see any air bubbles, then do the same thing to the front brake line.
As long as the brake pedal is pushed down very slowly, no air will be pushed down the steel lines. The fluid will go down, and the air bubbles will float back up. When the pedal is released quickly, any air bubbles that stick to the lines will be drawn back up to the reservoir.
Tighten both nuts, then wash the area with brake parts cleaner. Once the area is dry, hold firm, steady pressure on the pedal for about a minute. If it sinks slowly, there's a leak. Whether it leaks or not, check the fittings. If they're dry, fill the reservoir. Since you just installed new front pads, all the fluid that was in the calipers got pushed back up to the reservoir. You can fill the reservoir almost full because the level can only go lower as the pads wear.
A few additional notes: All containers of brake fluid must remain closed except when pouring to prevent absorption of moisture from the air. The moisture promotes corrosion of metal parts, and it lowers the boiling point of the fluid. Brake components can easily get over 212 degrees and this heat will transfer into the fluid. When the moisture boils, it makes air bubbles which compress causing a mushy brake pedal. The caps should remain on the reservoir except when filling or checking the fluid.
Moisture has a way of finding its way into the hydraulic system through seals and porous rubber hoses. Some people change the fluid every few years to prevent corrosion of metal parts. The best way for a do-it-yourselfer to do this is to open a bleeder screw, then let it gravity bleed for a few minutes. To prevent the heartbreak of a mushy pedal, you must never allow the reservoir to run dry. Professionals will suck out most of the old fluid before refilling with new fluid. That reduces the diluting of the new fluid with old, and the need to run a lot more new fluid through the system. Old brake fluid normally turns dark brown from being hot. When you see clear fluid coming out of the bleeder screws, you know most of the old fluid is out of that line. If the fluid drips very slowly or stops flowing, vacuum has built up in the reservoir. You will need to loosen either one of the caps to relieve the vacuum. You will also find that the rubber bladder seals under the caps have pulled down. This normally happens as the fluid level goes down when the front pads wear. So this can happen normally, you should push those bladders back into the caps. Be sure your fingers are clean when you touch them. Oil and grease on your fingers is one of the most common ways brake fluid becomes contaminated with petroleum products. Oil, transmission fluid, and power steering fluid will all destroy rubber hoses and seals.
Use a flare nut wrench, also called a line wrench, on the soft metal nuts. A regular open end wrench will tend to round them off. If only one nut is corroded to the steel line, remove the other line, then rotate the master cylinder to unscrew it from the line. Heat the stuck nut a little with a propane torch, then try working it loose with the line wrench. Tapping on it with a small hammer might help to loosen it up. Do not use any kind of penetrating oil or lubricant because the residue will get inside the line and contaminate the fluid.
If both nuts are corroded tight, loosen only one about one eighth to one quarter turn, then try tapping the steel line into the port. This might free it enough to work loose with the wrench. You might have luck heating the line where it goes into the nut, then quickly cooling it with a little water to shrink it. Avoid heating the nut itself because it's usually soft brass and the master cylinder is aluminum. Both of these will melt at relatively low temperatures. Chances are the two nuts will come loose quite easily because not much water and salt sprays up there to cause the corrosion. Corroded nuts are much more common on the lower line fittings by the rubber hoses.
Although unlikely, any air left in the lines near the master cylinder will usually work its way out from repeatedly holding the brake pedal down at stop lights. Rear drum brakes need a lot of fluid flow to move the shoes. The returning fluid brings air bubbles with it to the reservoir. Front disc brake pistons don't move very far so the volume of fluid flow is small. If you have reason to suspect air in the front lines, a quick trick is to remove the wheels, then use a flat blade screwdriver as a pry bar to push the pistons back into the calipers. Doing so will move a great deal of fluid into the reservoir, and the air will go with it. Then, be sure to pump the brake pedal again until the pistons move out to the rotors and the pedal gets solid. This is a good time to practice only pushing the pedal half way to the floor.
When this is all done, if you still have a low pedal, try pumping it quickly three or four times, then holding it down. If it pumps up and stays high and firm, release pressure for five seconds, then press it again once. If it now goes down too far again, the rear drum shoes are out of adjustment. When you pump the pedal rapidly, the master cylinder pistons take a new bite of fluid before the shoes have fully retracted and the fluid has returned. If you can't pump up the pedal under any circumstances, there's still some air in the system. You might have no choice but to bleed at the wheels, but I've never had to resort to that when I bled the lines at the master cylinder with a helper.
Wednesday, May 13th, 2009 AT 12:06 AM