[Supras] Caliper piston retracting

[berniek at technicaldevelop.com]

Hi.  Hope this is of some help.

    Since is past bedtime I did not look specifically at the TSRM, but many master cylinders or other elements in the brake system have rubber "duck bill" valves to not allow brake fluid back into the master cylinder with disc brakes after applying and removing the brakes.  The duck bill valve has just enough "give" to let residual runout in the disc cause the piston to retract.  The valve is there to let the piston remain in the last position within a few thousands of an inch, barely retracted from the disc.  This is why disc brakes to not require adjustment.

    Have you ever noticed that it takes quite a bit of force to make the piston retract when you install new pads?  That is due to piston friction to a minor extent, but if you feel the retraction screw or C clamp torque very carefully, the piston retracts at almost the same rate if you use little or high force.  I believe there is a small orifice in the duck bill valve which allows this reverse flow, so pressure drop across it when retracting the piston varies as the square of the force.  So you can apply more force but the piston retraction rate but it will not retract much faster.  

    When the duck bill valve prevents backflow under normal operation, the cup in the master cylinder generally allows a small amount of reservoir fluid to get sucked past it to make up the difference.  It is very small from use to use because the pads wear at a very slow rate.  If the master cylinder cup is stiff, it may pull a vacuum until it retracts to the compensating port connected to the reservoir, which allows the pressure side of piston to refill with fluid.  

    Under normal conditions, the duck bill valves act as check valves but with a reverse direction leak rate far slower than normal brake application-release operation will ever see.

    Hope this helps.

            BernieK