Subject: Weird brake question # 163
| Ok listers,
| The brake pedal is firm when traveling forward. When I go into reverse
| and then forward the pedal gets very soft and I have to pump the pedal
| to get pressure back up. After that all is well until the next time I
| have to back up out of a parking space. Any ideas? The entire brake
| system has been rebuilt (calipers, master cylinder, rear wheel
| cylinders, proportioning valve, hoese) and I am running Silicon DOT 5.
| I had heard that bad wheel bearing might be cause this kind of thing,
| but when you go around corners.
| Thanks in advance
| Jim Henningsen
Hi Jim,
Loose front hubs (loose bearings) can cause the pads & pistons to be knocked
back into the calipers causing the brake pedal to be soft the first time it
is pressed as all available fluid is used to push the pistons out to the
normal quiescent position. This can be tested by making a sharp turn in one
direction and then testing the brakes. Then make a sharp turn to the other
direction and test again. An alternative is to jack up the front and see if
one or both of the hubs are loose on the axel.
Since it only happens after being in reverse, my first guess would be the
rear brakes. Something may be hanging up the rear shoes when going forward
so than they never return. When in reverse, they may let loose and force
the piston back into the rear wheel cylinder. This then would require more
pedal the first time the brakes are applied again. The rear brakes would
also have to be out of adjustment for this to be the explanation. I'd apply
the handbrake firmly but not hard and see if the problem occurs when backing
up and then going forward. If the problem stops, the rear brakes need
adjustment. Also, one is probably hanging and that needs to be
investigated.
BTW, if you're talking about a TR6, that brass H type fitting below the
master cylinder is a Pressure Differential Warning Alarm (PDWA) device, not
a proportioning valve. It's function is to give an alarm if the pressure
between the front and back is different (proportioned).
Nelson Riedel
Granville, Ohio
'68TR250, '76TR6, '70TR6 (project)
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