Re: new brake booster seems like it's failing - Saabnet.com Bulletin Board
It is possible for a brake booster to fail without a vac leak. There is internal valving that can fail. The internal valving is responsible for the pressure differential between vac-side (front) and atmosphere-side (back) of booster. It's designed to let a little of the atmospheric pressure from outside the booster into the booster when you push on the brake pedal. This isn't enough to make the engine surge, but if it's not getting in (or if it's getting through to the vac-side) it could be a problem.
Other thing to check is the non-return valve (turbo cars only). Make sure boost isn't getting into your booster!
If you've ruled out the easy stuff, it's possible that the rebuilt unit you got has some problem within it. Internal valving on these is a bit of a black box. I asked Booster Steve (specialist of Dewey's Booster Exchange in Portland) and he said he usually doesn't take the internal valving apart unless absolutely necessary. He said the usual protocol is to inject grease, repair external seals, and test. It could be that Cardone did this, and the grease they injected filled up some pits in the steel or deformities in the rubber within the internal valving, and then it tested fine. Now that the grease has settled it's not working well anymore. Doesn't Cardone have a lifetime warranty? You might want to check to see if you could get your rebuilt unit replaced.
posted by 65.112.10...
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