Posted by Chris Case [107.201.30.23] on Thursday, August 14, 2014 at 23:46:19 :
In Reply to: Re: MC Repair kit posted by Doc Dave [70.192.194.168] on Thursday, August 14, 2014 at 21:24:39 :
It seals the perforated stamping against the end of he bore. The perforated stamping has a rubber check valve in it that lets fluid flow through to the wheels, but not back. To return, the fluid has to push the stamping off the donut. The spring pushes against the donut, which retains a small pressure. That pressure is called "residual pressure" and is just enough to keep the lips of the wheel cylinder cups sealed against the walls of the cylinder. Otherwise fluid could leak out and air in.
The little stamping with the holes is called "residual check valve".
Later dual cylinders have them too, but they are little things that look like brass seats for the lines.
Except disc sections do not. Disc calipers depend on the friction of the o-ring to return the piston just a skosh. So a residual check valve will heat and wear the pads. Plus, orings don't have the fine little lip of wheel cylinder cups.
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