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Posted by Chriscase @ San Diego on Thursday, January 03, 2008 at 21:56:27 :
In Reply to: Re: What's the deal with the residual pres. valve? posted by Paul (in NY) on Thursday, January 03, 2008 at 19:30:36 :
The 10 psi is not enough power to overcome the return springs on the shoes.
The cups used in wheel cylinders depend on positive pressure to maintain a seal. Without any residual pressure, fluid can leak out, and air can leak in, giving a soft pedal on the next application.
Disc calipers don't use the 'cup', they use an 'O' ring. Not only does this eliminate the need for any residual pressure, the residual pressure would keep the return-spring-less caliper piston pushing on the brake pads. Whereas, quite the opposite, the o-ring's friction and elastisity act as the minimal return spring, drawing the piston back slightly to it's original pre-application position.
On a single-piston master, the residual pressure is caused by the big return spring pushing on the aluminum check valve, against the big fat rubber washer that goes into the cylinder first. On the typical double master, the check valves go into the return line seats, allowing different pressures front and rear.
The big reservoir, usually to the rear, should be hooked to the discs. The larger capacity is used to fill the caliper as the pads wear and the pistons stay outward. Whereas drum system cylinder usually return to the same position, with an adjuster to take up the slack from wear. That said, the primitive adjustment system on the FFPW holds the drum outward, and the cups int he wheel cylinders are continually exposed to fresh bores- bores that have had months/years to become corroded. There has been progress in the drum brakes systems were invneted in the 20's.
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