TOPIC:

On automobiles, I believe it's pretty much universal to have a check valve in a hydraulic disc brake circuit that restricts fluid from returning from the slave cylinder(s) at the wheels to the master cylinder when the vehicle is sitting. This keeps the slave cylinders from retracting the pads away from the discs, so that when the vehicle is next used, the brakes will be instantly effective instead of needing to be "pumped up" before they're useful.

All of my street bikes have had front disk brakes, and obviously none have needed to be pumped up before use. What I'm seeing with the El-Cheapo $15 universal master cylinder is that I can achieve a good "pedal" on a given day, but when I come back the next day, it seems to have lost its prime, so to speak. Is this MC defective, or is there a check valve in the 1:2 factory distribution block that splits the pressure from the MC into the two slave cylinders that's stuck, or what?

rdurost wrote: On automobiles, I believe it's pretty much universal to have a check valve in a hydraulic disc brake circuit that restricts fluid from returning from the slave cylinder(s) at the wheels to the master cylinder when the vehicle is sitting. This keeps the slave cylinders from retracting the pads away from the discs, so that when the vehicle is next used, the brakes will be instantly effective instead of needing to be "pumped up" before they're useful.

All of my street bikes have had front disk brakes, and obviously none have needed to be pumped up before use. What I'm seeing with the El-Cheapo $15 universal master cylinder is that I can achieve a good "pedal" on a given day, but when I come back the next day, it seems to have lost its prime, so to speak. Is this MC defective, or is there a check valve in the 1:2 factory distribution block that splits the pressure from the MC into the two slave cylinders that's stuck, or what?

Can't speak about car brakes but motorcycle brakes definitely don't operate as you suggest. Motorcycle master cylinders have an equalizing hole in the hydraulic circuit to allow fluid to bleed back into the master cylinder when the brakes are at rest. The equalizing hole is required to allow for thermal expansion of the fluid and to accommodate increased rotor/pad thickness due to heat expansion (and to allow from some rotor 'warpage'). If the pads aren't allowed to float by having fluid flow back into the master cylinder, the brakes will lock up (which is exactly what happens if the equalizing hole get plugged - usually because the fluid hasn't be flushed frequently enough). The equalizing hole is located in front of the master cylinder piston so it's open when the brakes are at rest but when the brakes are applied the master cylinder piston moves forward and 'closes' the equalizing hole so that solid pressure is maintained - but only while the brakes are applied - maybe this is the 'check valve' action you're thinking of?

Friction keeps the slave cylinder pistons where you used them last (with the rubbers boots and seals pulling them back slightly) unless they contact the rotor which will push them slightly back into the bores so they're not in contact with the rotor when the brakes aren't on.

If you have to "pump" the brakes to achieved a "hard" lever then in all probability there's air in the brake lines.

The caliper boots and seals have nothing to do with removing applied brake application pressure. The spring in the M/C returns the this piston to "Home" and in doing so draws back the fluid - which removes the applied pressure to the brake- pads .
There is no "check-valve" anywhere in the system, nor can there be for what should be an obvious reason.
Regarding the "El Cheapo" master cylinders: When I converted to a Hydraulic clutch I purchased these as to have matching masters:

No issues or concerns. They work perfectly per application.

Minor point but your diagram has the equalizing/compensating port (smaller of the two ports) and fluid inlet port positions swapped (at least for modern motorcycles). And slave cylinder piston seals (square ones anyway) absolutely help with retraction of the pistons. You can verify this when bleeding the brakes: apply the brakes, when the bleed valve is opened, the pistons will retract slightly even though the master cylinder piston has not moved from it's fully 'engaged' position

Not my diagram ! Copied from the net. as with the Following:

I guess they're all wrong.
Not going to debate whether the "seals and boots" retract the pistons. For the purpose of clarity - The spring in the M/C returns the M/C piston - thereby pulling the caliper pistons away from the disc. when the lever application pressure is nil. Fluid Dynamics !

I had it backwards--it's DRUM brakes that use(d) Residual Pressure Valves, supposedly to counteract the retraction springs pulling the shoes too far back away from the drums.

There are a few disk brake applications that use RPVs, but apparently most don't need them.

www.speedwaymotors.com/Speedway-Disc-Bra...PSI-Purple,5312.html