Sure. The old MC was indeed bad, and most likely did allow air into the system, but if you don't bench bleed a master cylinder before installing it, it will pump a boatload of air into the braking system. Air rises. If you have pedal pressure and there's air in the system, those air bubbles will rise back up the lines into the proportioning valve and the MC, making the pedal pressure inconsistent. The diameter of the MC is different than the brake lines so the amount of air in either one will make the pedal pressure different. Air compresses, brake fluid does not. If you're mashing the pedal and you can't lock up the brakes, 99% of the time there is air in the lines.

You don't need to bleed your brakes once or twice. You need to bleed them until ALL of the air is out of the lines, and you need to do each wheel cylinder in the right order or you won't get it all out. TOY4TWO posted that order, and can attest to how much of a PITA it is without bench bleeding. It's a lot more work, sure... but if you're going to do this kind of work yourself, you HAVE to do it right and get all the air out or you're putting yourself and other people in danger.

At this point, you're better off not removing it and bench bleeding it because there's already a ton of air in the lines. You're going to have to bleed that out anyways, so pick up an extra bottle of brake fluid, start bleeding, and don't let that reservoir run dry.

 

bled the snot outta the whole system. waited a week and did it again. still causing problems. there is no way there is air in any of the lines. another person said he had similar problems with an older vehicle of his and it was the booster. looks like i get to change this now.

 

did you bench test the MC? before you put it in? Have you pulled it out and bench tested it since the install? bench testing the MC would cross it off the list of possible problems. Then work your way down the system, one line at a time, to diagnos

 

I'm not saying this will completely rule out any possibility that something is up with the brake booster, but to check that it's functioning correctly (at least at the moment of the test) here are the steps:

First thing is you press the brake pedal several times (w/ engine off) to build pressure and then while holding your foot hard on the pedal, start the engine. The pedal should sink slightly, if it doesn't sink at all it would indicate a problem with the booster itself or it's check valve.

To test the booster for leaks: Press on the brake pedal while the engine is running (not sure if it matters that it's at normal, warm idle) and then turn off the engine while continuing to press on the pedal. The pedal height shouldn't vary, if the pedal starts to rise it would indicate a faulty booster.

To check that the booster check valve is working, press the brake pedal several times with normal pressure while the engine is off. It should start off low and gradually rise as you keep pressing it over and over again. If the pedal height doesn't change, you should check the booster check valve directly.

To do that, disconnect the booster vacuum hose where it attaches to the booster itself. Start the engine and let it idle, you should be able to feel vacuum. If not, the check valve isn't working.

 

BTW, I didn't see this posted above, but I might have missed it...

Have you ever checked to see if the brake system (MC, lines, calipers) is actually holding pressure when you aren't driving and the engine is off (taking the booster out of the equation)?

With the engine off, you just press the pedal over and over until there is pressure and then you press hard on the pedal and hold it there for 1/2 a minute or so. The pedal should stay firm and not drop. If it does drop, you might have a leak somewhere (could be real small) that is letting air in, but not letting enough fluid get out for you to notice the difference :dunno:

 

ok looks like i got some new things to try out here. will let you know how testing goes once i get to it. thanks for all tech advice.

 

'hate to tell ya this, but it ain't yer vacuum booster. All that does is amplify the pressure to your brake master cylinder, and if it failed, you should still be able to lock up your brakes by mashing really hard.

http://auto.howstuffworks.com/power-brake2.htm

If the vacuum booster failed, the pedal would just get really stiff. It won't fall to the floor unless either the fluid leaks somewhere, or if air is coming back into the lines somehow and compressing when you mash. It has to be leaking somewhere. Mark your brake fluid level, and after your car does this crazy shit :loco:, park it back where it was when you marked it (to ensure your car is at the same level) and see if the fluid level has dropped any. I bet you'll be surprised at what you find. Perhaps the adjustment of the brake master cylinder rod is not right?