Quote:
Originally Posted by ModelA29
With disc runout the pads can just keep backing off (there is no positive stop and the runout forces the fluid back into the master cyl) needing you to pump the pedal to get them back into contact. Drum brake retraction is mechanical and they have a positive location when the springs retract them. If you need to keep pressure on the wheel cylinders the cups are worn. To check that idea just open a bleed valve on the wheel cylinder if there is pressure the fluid will squirt out. Some people will add a valve to the rears also but I've never encountered a situation that needed one.
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Well, rotor runout will only push the pads back the same amount as the runout. I've test driven a lot of cars with rotors warped so bad that the steering wheel would shake violently when applying the brakes, yet I never needed to pump the brakes to extend the pads back to the rotor! Having to pump the pedal indicates some other problem.
I would suggest that you do an internet search for "Residual pressure brake valves".. You will find a lot of information on the need for residual pressure valves on drum systems (as well as some disc systems). The pressure maintained in the system is only 2 to 10 lbs. so you won't get much of a 'squirt'. Please note that the real experts mention several reasons why residual pressure is needed.