Quote:
Originally Posted by 40 Deluxe
Oops! I think that you have it backwards. On a disc brake system the only thing that retracts the pads is the slight deflection in the square-cut 'O'-rings that seal the caliper pistons (there are no return springs). When the brake pedal is released, the 'O'-rings relax and pull the piston back just enough to prevent drag. If a residual pressure check valve is used (usually about 5 lbs.), the pads won't fully release and will drag.
On a drum system, the cups in the wheel cylinders will leak unless about 5 lbs. pressure is kept in the system. Thus a residual pressure valve is needed. The shoes will not drag because the shoe return springs easily overcome the 5 lbs. residual pressure.
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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.