Brake Booster Trouble

[Jon I]

After spending months trying to track down a “pop” when I pressed the brake pedal on my ’91 C2, I finally traced the problem to a worn rubber diaphragm in the power brake booster. I bought a replacement unit from the Porsche dealer, installed it, and bled the brakes, all without any problems. I took the car out for a test drive, pressed lightly on the brake pedal, and was almost thrown through the windshield. The “pop” problem was solved, but now my brakes were so sensitive that it was hard to do anything short of a max braking stop.

After several more hours of checking the installation and re-bleeding the brakes, I gave up and took the car into a local Porsche specialist. They flushed the brake fluid, bled the system and double checked my work, but weren’t able to solve the sensitivity problem and were at a loss as to what was wrong.

I chalked the problem up to a defective booster, so I had the Porsche guys get another unit shipped in from Germany. I installed the new booster, bled the system, and gave it a test drive only to find that the problem is still there. Has anyone ever dealt with a problem like this? Here are some other observations:

- The brake pedal depresses with very little resistance, but the brakes engage normally. I guess I would describe it as ‘over-boosted’.

- The original booster was made by Gerling, but both replacements were made by Lucas. Maybe the Lucas units aren’t made to the same specs? I had the dealer double check the part number just to make sure that I was given the correct part for my car.

- Other than this problem, the brake system seems to be in perfect condition.

Any suggestions?

[Jon I]

...still no luck fixing this.

Is there a way to adjust the vacuum pressure feeding the brake booster?