That leaves out rear calipers out of adjustment. I just looked and I see your truck never came with rear disc brakes. One thing to look at if the fluid is relatively low in the reservoir is to have a helper push the pedal somewhat hard and quick, and watch the fluid with the cover removed. You should see two spurts of fluid shooting up with each pedal stroke. That will verify both pistons in the master cylinder are able to build pressure even if they do have a minor internal leak starting to develop.
If you don't get those spurts, loosen the mounting bolts so you can pull the master cylinder 1/8" away from the power booster, then try again. If you get spurts now, something is holding the pistons partially applied. That will prevent them from taking on more brake fluid in front of them. Some vehicles have adjustable push rods coming out of the front of the booster but that usually isn't the problem unless someone has already misadjusted it. A more likely cause is a misadjusted brake light switch. Your switch is self-adjusting and will not get out of adjustment unless it is physically put that way. It still should readjust itself.
Did you bench-bleed the new master cylinder before installing it? There is a simple trick you can do that eliminates the need to bleed at the wheels. If you installed the new master cylinder dry, there will be a whole bunch of air to get out. That is made extra difficult on a lot of GM vehicles where the master cylinder sits at an angle, raised higher in the front. Air can collect in the high spot and never bleed out. The air will not get drawn back into the reservoir because it has to go downhill to get there, and it won't get pushed down to the wheels because due to the relatively low velocity from pedal bleeding and gravity bleeding, the air stays in the high spot and fluid just flows past it.
To check for that trapped air, loosen the steel line nut at the front of the master cylinder a quarter turn, then have a helper slowly push the pedal halfway down. Tighten the nut before the pedal is released or air will just be sucked back in. If air is present, the bubbles will be obvious. Do that two or three more times until no bubbles come out, then do that to the rear line.
GM likes to use coiled steel lines going down from the master cylinder to reduce problems from vibration. That results in the air having a longer way to go and more chance for it to keep floating back up while the fresh brake fluid is going down. What often works is to apply the brakes slowly, wait a few seconds, then let the pedal come back up quickly. Pushing slowly pushes fluid down without pushing the air down further. Releasing it quickly makes the fluid rushing back to the reservoir carry the air with it. That air may collect again in the front of the master cylinder and have to be bled at the lines. That trick works on any car and is one reason why often the brake pedal will feel better after a few days.
If the pedal only sinks too low while driving but it pumps up solid when standing still, suspect loose front wheel bearings. As the brake rotors wobble, they push the pistons back into the calipers further than normal. To get them back out to where pressure can be built up requires pushing the pedal too far.
Rear shoes out of adjustment are common but since you already checked that, also look for sticking parking brake cables. That will prevent the shoes from adjusting later. To check for that, both shoes must be touching the large anchor pin at the top of the backing plate, and you should be able to push the parking brake strut rod a good 1/8" against the anti-rattle spring pressure.
If you're still having trouble, we have to wonder how the air got in the system in the first place. Failure of the residual check valve, which is just about unheard of, will allow air to seep into the rear wheel cylinders when barometric pressure goes up. The clue would be you would repeatedly find air when you bled them.
Check the rubber hoses for cracked outer casings. It's not common, but if the reinforcing strings are broken, the hose can balloon up creating a loss of pressure.
Friday, June 10th, 2011 AT 5:31 AM