front brake question, XS2

[tmikeyb]

I now have plenty of front brake after putting new seals and pistons in my XS2 caliper. Since the pistons are a pretty snug fit, I'm just curious about piston retraction when the brakes are released. I like to not only fix things, but I also like to know how they work. I'm assuming that the pistons don't actually retract much when the brakes are released. Maybe just enough to release the rotor. Is that correct? Do the pads actually lightly touch the rotor when the brake is released. I just don't see any pressure or suction that would be adequate to retract the pistons. Please educate me. THANKS

 

[Jim]

The master cylinder is actuated by the rider and returned with an internal spring. When you move the lever, the first thing that happens is the piston closes the compensating port. Now all pressure goes down to the wheel cylinder, forcing it out and actuating the brake.

When you release the brake lever, the spring inside the master retracts the piston back to the brake off position. Since it's a closed system, that also pulls fluid from the wheel cyl, causing that piston to retract... or move away from the pads... it sucks it in, if you will.

Once the compensating port is (re)exposed, it loses all pressure (suction) and allows the reservoir to replenish the fluid in the cylinders and lines... it "compensates" for pad wear.

So yeah, it moves the pads away from the disc ever so slightly... enough that there's little to no drag on the disc. You might feel a little drag when you spin the wheel by hand, but since it's not pressurized (open compensator port), it doesn't actually apply any pressure to the disc.

 

[gggGary]

The rubber seals in the caliper also tend stretch/move with the piston, rather than release, so when you let off pressure the rubber acts to pull the piston back a bit.

 

[Mailman]

I know that whenever I rebuilt front discs ( new pads and scuffed discs ) you can hear the drag on the disc until that disc gets shined up again.