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Brake booster question 700 1989

posted by  someone claiming to be Garrett Kirkland on  Thu May 24 18:06 CST 2001 [RELATED]

I decided to stop pulling my hair out over a low rpm stalling problem and take my car to the dealer. They said that my brake booster was bad and I had a short in the throttle position sensor circuit. The hair-pulling part is that I replaced both parts trying to cure the problem before giving up. I replaced the throttle position sensor and didn't even consider a short in the circuit. I bought the brake booster used and apparently swapped a bad one for a bad one. I knew the old booster was bad because I could hear hissing when I pressed the brakes hard at a stop. When I took it out, you could move the plunger in and out and hear the leaking bladder. But I currently don't have any symptoms except bad brake pedal feel (feels like air in the line) and the aforementioned stalling which I thought was unrelated. Before I installed it, I checked to see if moving the plunger would pressurize the booster. After installing, I noticed when removing the vacuum line that there was significant vacuum at the booster.

My questions are these: How do you test a brake booster to tell if it is bad? Is the brake booster in an ABS car the same as the one in a nonABS car? (I have an 89 760 Turbo with ABS.)

Thanks for your help.







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    Re: Brake booster question 700 1989

    posted by  someone claiming to be L K Tucker on  Fri May 25 06:11 CST 2001 [RELATED]

    /// Sounds like you have my luck. What was Al Cap's character that had the black cloud always over his head? I can't remember the spelling and am to lazy to look it up. You pronounce it by making the rasberry sound. Joe Bizzzzth?

    One test is to remove and plug the vaccum hose. If the engine idle trouble stops the booster probably is the trouble.

    If you install a valve and vaccum gauge, you should be able to close off the booster with vaccum. Watch the gauge, if the diaphragm is cracked the gauge will not stay up. As soon as you close off the vaccum from the running engine the gauge will fall. There are hand pumps and AC vaccum pumps you can use to test vaccum.







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      Re: Brake booster question 700 1989

      posted by  someone claiming to be Art Benstein on  Fri May 25 11:25 CST 2001 [RELATED]

      My daughter's car (83 DL) would sometimes stall when about to move after starting: You'd put your foot on the brake, move the shift lever to drive and releasing the brake would drop the rpm.

      I had checked the booster for leaking using a vacuum tester - it held. I used a more primitive test on the check valve (after wiping it as clean as I could), but didn't notice any problem. Had just bought a spare check valve for another car, so I decided to try it. That did the trick. No obvious cracks and the grommet was still supple -- guess the the old mouth-test's not as reliable as I'd figured. I now bring that vacuum tester any time I'm looking for diaphragm operated parts in the junkyard.







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        Re: Brake booster question 700 1989

        posted by  someone claiming to be L K Tucker on  Sat May 26 15:34 CST 2001 [RELATED]

        /// Your problem as you posted was on release of the brake. The check valve is not involved in this and the other instance I was discussing. The check valve keeps the vaccum in the booster in the event the engine stops with the throttle open, and will always open if the vaccum in the booster is not as high as the intake manifold vaccum. Even if you took it out, provided the booster is not damaged, there would be no effect on braking or engine rpm.

        Vaccum boosts the pressure on the master cylinder, then it must release that pressure, when the foot is off the pedal, by venting in outside air.

        I am not an expert on the internals of the Volvo booster. The vaccum must be eliminated without allowing more outside air in than is necessary to allow the plate and diaphragm to return to rest. When the valve or device sticks open on release it provides a path through the booster for unregulated air. If the diaphragm is cracked then application of braking will cause that outside air to get to the intake.

        Again I have never actually cut a Volvo booster apart. All my junk ones work.






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