1990lxhatch

Replace your brake booster. When you step on the pedal, you open a valve that allows the vacuum in the rear of the booster to "vent". A more accurate description would be that air under atmospheric pressure is allowed to rush in. The vacuum on the front half of the diaphram along with the ~15psi of atmospheric pressure on the rear half is what reduces the pedal effort. HOWEVER, the amount of pressure allowed to rush into the rear half is metered by your pedal position.

If you have a vacuum leak ONLY when on the brakes AND the brakes lock up with the slightest amount of effort it sounds like you have either a crack in the rear half or your pushrod isn't set properly. If you did an auto/man conversion you obviously were messing with the pedals.....

The best test I can suggest would be to make sure you have no obvious contamination on your brake pads/rotors, check their operation with the wheels off the ground and disconnected (and plug) the vacuum supply to the booster; retest. If something is up with the hydraulics or calipers your symptom should be there with the power assist bypassed. Obviously the pedal will be stiffer but hopefully you know your car well enough to tell the difference.

My $ is on the booster

93stallion302

thx for the advice, Ill give that a try this weekend, your explanation is specific enough that I should have a diagnosis pretty fast. Thanx again.

Tony71502

Moisture in the brake fluid will not cause these problems.... The brake fluid has to be hot enough from severe braking for the water in the fluid to BOIL in order to cause problems....