78Silver928

Per the title... I replaced the brake booster and master cylinder in my '78 last fall and did the clutch master at the same time. The brake booster was a used unit from 928 Intl. At the same time, I rebuilt all 4 calipers and replaced the pads. Ever since getting the car roadworthy, the brakes just aren't performing well. Takes a lot of pedal effort to stop the car in most cases. Sometimes it seems like the booster is working at low speeds with gentle effort, but a panic stop is just that... a panic. I can't seem to get the brakes to lockup at all no matter how hard I press on the pedal.

I did some investigation with a vacuum gauge. At fast idle, I have 15 in vacuum at the booster hose. The check valve seems to be working. None of the hoses are leaking. I put my mightyvac on the booster port and can't get any vacuum built up. I have seen other posts about the O ring between the master and the booster... and I'm not certain I replaced it. I remember the job being a huge PITA though and REALLY don't want to start all over, especially since I only just recently got all the air out of the clutch. Question: Is it possible to pull the master cylinder away from the booster enough to put a new O ring on there without removing all the fluid and taking lines loose? Would be worth a check to see if it's just the O ring, or if it is even there. The other alternative is a faulty booster, but I'd love to rule that out. Any thoughts?

The Forgotten On

If your master is original you should replace it. Your booster is probably fine otherwise.

The brakes on my 81 sucked even after I replaced both the master and booster. I made sure I bled the brakes perfectly and they were still disappointing.

I could only lock the brakes under 15 mph. Otherwise it was completely hopeless in an emergency. So I upgraded to 993 brakes, the car is so much easier and safer to drive after that.

78Silver928

Master cylinder is brand new. Original booster was full of brake fluid and rust holes.

michael j wright

As explained to me by a master tec and verified by an A1 Cardone rep. The best way to test a booster is as follows. 1. With engine off, pump brake pedal till it is very hard and will sit high. 2. While foot is still on pedal, start engine and if the pedal goes down then booster is fine. This shows two things. When pumping pedal and it gets harder, had vac and it was holding. Then when starting it is able to have vac at the booster. Also was told any noise you hear in cab while pedal is depressed is normal, just the felt seal designed to mask the noise is wearing out and is of no concern.

Mrmerlin