Brake Sympton, Firm then gets mushy when engine starts

theclaw · 02:11 PM

Got a funny brake symptom.

I just installed new rebuilt calipers on from Eric at PMB.
Flushed/Bled the system thoroughly twice.
On the road, it takes a pump or two to get them firmed up.
WITH THE MOTOR OFF - I can pump the pedal a couple of time to firm it up.
TURN THE MOTOR ON - the pedal gets squishy and it looses pressure.

Any ideas? Vacuum boost? I'm going to Blackhawk this weekend, and traveling for work most of the week. Time is tight .....

Brake system is stock, 1984 Carrera

Thank you,

spuggy · 06-16-2013, 02:22 PM

Time-honored test for a functional brake servo is to pump the pedal up hard with the motor off, keep pressure on the pedal and start the motor. If the servo works, the pedal will drop slightly as vacuum is created. It then usually feels less firm. Normal for any car with a vacuum servo. If it's not working, there will be no change as the motor starts...

Sounds to me as though either the seals on the M/C are suspect (as they pump up) - or there's still air in the system.

If you manually bled the system (e.g. by working the brake pedal with the bleeder valves open), you should know that M/C can get damaged by extended strokes beyond the normal working range - if that part of the bore has corrosion, this can damage the seals.