In the process of going through my car, I have recently replaced the rotors (all), the brake lines (all), the master cylinder and the brake pads. Since that time I have had several incidents of brake drag, where the calipers do not release. In some instances the car is virtua;;y undriveable, and the rotors and rims are extremely hot. I have re-bled the master cylinder, which required some 1 to 2 ounces of additional brake fluid. I'm not sure I didn't boil it off when the calipers locked the first time. The problem is intermittant and now much less pronounced than before, but not eliminated. Any ideas as to what I am missing? Is there another pressure regulator of some sort in the system that need be replaced? Thanks in advance.
1981 928 5spd.
I had what sounds like a similar problem on my 1972 911 in which the rear rotors and wheels would become very hot. It turned out that the old rubber hoses had deteriorated and the passages had become partially clogged. Under the high pressure of applying the brakes all was well, but the pressure could not drop when the pedal was released. The brakes were in essence, always on. I was very dubious when this was suggested as a possible cause of the overheating, but cut one of the lines in two and by golly, the hole was clogged with rubber! New braided stainless lines took care of the problem cheaply and relatively easily.
Hope this helps.
1989 S4 blk/blk
I am sure others will give you things to check, all of them valid. The thing that comes to mind as the most probable is the adjustment of the brake pedal to master cylinder rod. There is a bleed hole in the master cylinder which connects the cylinder to the reservoir. in order for the system to work properly the master cylinder must be allowed to come all the way back to the end of travel to uncover this hole. If the rod is adjusted too long this wont happen. then when the fluid in the calipers gets hot it expands and you get a build up of pressure which stops the rotors from turning.
1978 928 5-speed
Guards red/black/tan sheepskins
I recently posted a problem with brakes seizing on the car as it was driven. I recieved several suggestions for diagnosis, all very helpful. We followed out all and eliminated the obvious. We were particularly concerned since rotors, pads, master cylinder and brake lines had all been replaced. The end result was a failed brake booster. Apparently, there has been some problem over time resulting in brake fluid accmulating in the booster. The diaphram in the booster had failed. According to my mechanic booster failure in the 928 is not usual. For those of you who face a similar problem in the future, eliminate the booster failure as part of your daignosis. At least I will get the beast back for the long weekend. Thanks to all who took the time to help.
1981 928 5 spd
I am so glad you found this problem. I had the same thing when I rebuilt my MC and had completely forgotten about it. Didn't get to the point of the brakes seizing but it is good to point out to other owners. In my car it wasn't a breached booster diaphragm it was the ATE master cylinder!
WARNING, old brake master cylinders can fail in a wierd way. There is a double seal at the back of the ATE master cylinder where it mounts to the vacuum booster. When this seal set fails the vacuum of the booster literaly sucks your brake fluid into the front (towards the front bumper) half of the vacuum chamber. If enough fluid collects there it can hydraulically lock the master cylinder and that acts just like you were standing on the brake pedal all the time. You must replace or rebuild your master cylinder to repair this.
If you are pouring in brake fluid and there isn't a puddle on your garage floor but your brakes are still working fine, your time is nearing!
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