Sticking and Overheating
>The brakes on my 87 5sp seem to be getting worse. I first had a problem
>last year on the way to Sharks in the Harbor, which is a 3 hour drive.
>After about 2 1/2 hours of driving, both front calipers began to apply
>themselves at highway speeds and I found myself looking for a way to cross
>6 lanes of traffic with a rapidly decelerating car. By the time I reached
>the safety lane I was floored in 2nd gear and the brakes were smoking. The
>brake pedal was completely at the top and would not budge. After sitting
>about 15 minutes, everything was fine and I completed my trip. On the way
>back, I kept the speed at 60 or below and the brakes were fine. I replaced
>the fluid and figured that took care of it. 6 months later on my next
>3-hour trip the same thing occured. But this time, after cooling down the
>pedal is noticeably spongy and must be pumped once after the car stops to
>keep it from sinking to the floor. I replaced the rotors and pads, bled
>the brakes and refilled with ATE Super Blue. But, the fading hasn't gone
>away. What should I try next? Master Cylinder?? The problem seems
>consistent and manageable, so I've continued to drive it around town. Is
>this a bad idea?
Sounds as if the brake pedal push rod is misadjusted.
In the master cylinder, there is a very small hole in the side wall of the cylinder, called the bleed hole. This hole should be located just in front of the piston cup. When the pistons and cups are in the fully released position, this exposed hole allows fluid to pass freely from the cylinder to the resevoir and vice versa.
If the brake pedal push rod is misadjusted so that the holes are covered up by the piston cups, the fluid is trapped in the system. Heated fluid expands, lightly applying the brakes. This heats the fluid, which applies the brakes more, which heats the fluid more, which .... - You get the picture.
Another possibility is that you have a bad flexible brake line that is acting as a one-way valve, but that usually affects only one wheel on a 928. I think that the pedal rod adjustment is more likely.
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