Brake Booster Trouble???

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Mar 6, 2003
Arlington, TN
Would a slight leak in the vacuum coming from the brake booster be enough to make me have to pump the brakes twice to get the cruiser to stop??? Or, do I have another problem I need to look into? I have put all new wheel cylinders and shoes on, and a new master cylinder and braided lines. The brake have been adjusted, and bled and I still have to pump them twice!

Thanks :-/
according to my mechanic (who specialize in brake work), that could be exactly the case. in my situation, i replaced the master cylinder, adjusted the brakes, and bled the whole system about 5 times, and still had to double pump. Thinking i just bled poorly, i took it to the shop, and i was told that my brake booster was leaking and causing the problem. additionally, my old m/c turned out to be good, so there went a cool 170$! The moral of the story is: check for that leak, and good luck
Thanks, I also have bled and bled and bled. I am so frustrated. I found the leak yesterday while checking all the vacuum lines going in the carb. It also ran alot better with the vacuum line pinched off, but I had MANUAL brakes and couldn't get the darn thing to stop very easily. I will look into getting a new booster.

Anybody know the ins and outs of the check valve that the vacuum line attaches to on the booster? I didn't know if this could be causing the problem.
Stick your ear down below the steering wheel with the engine running. If it is the booster and the leak is constant, you will hear a hiss coming from the firewall. If you don't hear it that way, push on the brake, and see if you can get it to come in that way. A bad booster will usually leak toward the back where the pedal linkage enters the booster. I haven't heard of the check valve being the problem, although I suppose it could be. See Bailey's note elsewhere for booster cures.
Maybe i'm just not understanding it right, but could someone please explain how exacly the booster has anything to do with having to pump the brakes twice? As far as i can work out, if the booster has a bad leak, but the hydraulics work, the pedal will push in to a certain amount and slow the car down, but the pedal will begin to feel firm untill there is no vacuum left.

Even if the booster is completely stuffed, it becomes a basic mechanical straight through push - like no-booster systems. And having to double pump the pedal just means there is something in the hydraulics compressing more than it should, so the brake pads are not reaching or providing enough presure to their respective contact surfaces, and the second pump adds more fluid to the system from the reservior increasing pedal height.

You did bench bleed the master cylinder fully before you put it in the car didn't you? Because its almost impossible to bleed the master cylinder once its been fitted.( well for me anyway) This sounds like it may be your problem, because even with a stuffed booster the pedal should not sink to the floor, and should not change heights after pumping. It should just be a lot firmer to push.

I bench bled, adjusted, and bled the brakes aoout 10 times. I have all new brake components from the master cylinder down the the retainer springs. I am just at a loss and I am getting very frustrated. It works fine as long as I pump twice. I am getting to the point where I am just going to take it to a BRAKE shop and tell them to FIX it and call me when it is done.

Thanks for everyone's input. It has been helpful.
Agree with Sam, a leaky booster will not make you pump the brake pedal twice to stop. One thing people miss when they redo their brakes is adjusting the rear brake shoes correctly. When the shoes aren't adjusted right you will have to pump the pedal several times to fill the rear brake cylinders enough to make the shoes contact the drum.

If you have self adjusting rear brakes (81+) make sure the parking brake cable is tight or the auto adjuster won't work.


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