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BRAKES fine when moving. Pedal Vibrates when at stop. ABS or Master Cylinder?


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My brake symptoms only occur when at a stop. Brakes function perfectly when car is in motion. Only when the car has come to a stop and with my foot still on the brake pedal, sometimes the pedal will drop slightly and I will hear and feel a "buzzing" vibration underfoot. My car is a 1991 Legacy Wagon, automatic with ABS brakes. It is the ABS unit that is vibrating (removing the ABS fuse stops it), but could it be another component of the system causing it? Sometimes I can wait at a traffic light with my foot on the brake quite firmly and it doesn't do it. But recently I was experimenting, and I found that by moving the shifter away from "Drive" I could provoke the symptoms immediately (again this is done while at a stop and with my foot on the brake pedal). I'm questioning if a sub-performing master cylinder can possibly be at fault, causing the ABS system to kick in. I think the brake pedal travel might have increased gradually over the past year. It could be fine but I'm open to the possibility that I've just gotten used to more pedal travel. A simple diagnosis described in my Haynes Repair Manual suggests that the vacuum boost is working fine. Any thoughts would be much appreciated...

Edited by jodar
just did another diagnostic and wish to adjust the post accordingly
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