Power brakes not working

Kekec944/84 · 12-09-2005, 07:01 PM

The car is a 79 928.

The problem is the power brakes are not working. When I press on the brakes there is a hissing sound. Its comming from the area where the brake goes through the firewall, where there is that boot for the brakes. There is no sound when the brake is up, but when I hold the brake for example at a red light it hisses, and if I depress it further, with a lot of effort, the sound goes away.

Today I bled the brakes and flushed out the old fluid. When I took the car for a test drive the brakes worked properly, I parked it then drove it again a few minutes later and the problem was back. I thought the problem was bad vacume hoses to the booster but why would it work that one time. I'm going to check vacume at the booster and the check valve? Is there anything else I should try or check?

Whats that part called where that hissing is comming from? I'll try to get a picture up soon.

928ram · 12-10-2005, 07:05 AM

There is a port on the driver's compartment side of the booster that allows air to enter and exit the back side of the booster, there was a foam "muffler" installed back here which may have rotted away over time - thus the sound.

I'd look at the check valve, if it's failed or weak the booster won't be pre-loaded by vacuum which will require a longer stroke and a longer hiss. Also if the check valve has failed you should see better brake boost under high vacuum condition such as idle or decel, and poor or no boost under accel conditions. Poor overall vacuum will also affect it (leaks).

The hiss should stop after a short time if it's just normal air displacement; if there are times it just never goes away, the booster has probably failed.
If in the end it's a bad booster, be aware that the early '79s used a 9" booster which is readily replace by the much more common and cheaper 10" used from mid 79 thru 83(?). Any year 10" will fit, but later ones used a different internal ratio which may make brake feel a bit stiff or soft (but functional). I just replaced the 9" on my '79 with the 10" and it's a major P.I.T.A., so allow yourself some time if you need to do this.