Brake Pedal Sags

Brake Pedal Sags

I have noticed that while sitting at stop lights in my '82 AT, the brake pedal starts to sag ever so slowly. The car is idling in drive. Sometimes it sags enough that my foot pressure on the pedal decreases enough to let the car edge forward. I have performed the tests recommended in the 928 International catalog for checking for leaking master cylinders and mine seems fine. I replaced the vaccuum lines to the brake booster with no effect. All other vaccuum lines in the engine bay have also been replaced. Do other auto trannie owners experience the same brake sag with constant pressure on the pedal? Is this just a peculiarity of the car or can anyone recommend a cure for it?
Thanks in advance
John McDermott 1982 928 AT

The master cylinder needs replacing. This is a condition known as "by-passing". The pedal rod pushes on two rubber cups mounted in tandem (one after the other) in the master cylinder bore. When the cups and the bore are good, the pressure causes the cup to flare slightly, sealing the fluid in front of the cups, building up pressure that applies the brakes.

When the cups or the bore wear slightly, the cups don't flare and seal, and the brake fluid by-passes the cups, allowing the pedal to sink slowly. It will get worse.

Wally Plumley
928 Specialists

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