Topic: Is my caliper piston useable????

[TwoTired]

A pitted piston WONT CAUSE BRAKES TO RUB.
 It may leak but it wont cause brakes to rub/drag.
 Bad adjustment or set up causes that problem.
Even if brakes drag, when pressure gets too high fluid returns to master cylinder

I have to disagree with with you on this one, pj.

Although a blocked return will certainly prevent hydraulic pressure release, and proper adjustment is required, that is not the force that withdraws the piston from the disk.
The reason why the caliper seal is a square section seal, is that on brake application the seal square section goes parallelogram before sliding.  When fluid pressure is released, it returns to square, withdrawing the piston enough to lose brake pad contact with the disk.   The piston doesn't really slide on the seal until pad wear requires it. 

When the pits in the piston get large enough, and are under the seal, the seal fills those pits and changes the geometry of the square section, reducing its piston retracting effectiveness.  A few small pits have less debilitating effect as as many large ones.
Therefore, when the pits get bad enough, the seal is no longer a square section and the pad rubs the disk.

I've seen badly pitted pistons where the seal still retained fluid properly.  But, after a time period of a week or month, the brakes would then drag.  Repositioning the seal would cure this... for about a week or month.  And then, the problem returned as the seal took a set in its new position in the corrosion pits.

Cheers,

[crazypj]

The reason why the caliper seal is a square section seal, is that on brake application the seal square section goes parallelogram before sliding.  When fluid pressure is released, it returns to square, withdrawing the piston enough to lose brake pad contact with the disk.   The piston doesn't really slide on the seal until pad wear requires it. 

When the pits in the piston get large enough, and are under the seal, the seal fills those pits and changes the geometry of the square section, reducing its piston retracting effectiveness.  A few small pits have less debilitating effect as as many large ones.
Therefore, when the pits get bad enough, the seal is no longer a square section and the pad rubs the disk.

I've seen badly pitted pistons where the seal still retained fluid properly.  But, after a time period of a week or month, the brakes would then drag.  Repositioning the seal would cure this... for about a week or month.  And then, the problem returned as the seal took a set in its new position in the corrosion pits.
Cheers,

 As you say, the piston in effect 'ratchets' out due to pad wear.
Initially though, the pads over-retract.
The amount of rubbing is never severe enough to cause wheel lock up, usually it only glazes the moving pad.
 Almost all the Honda calipers I've looked at 9 and Yamaha ones) have the bottom of the seal groove machined at a slight angle (about 5~10 degrees?) the 'inner' edge is higher than the outer, the seal is angled when fitted and the piston slides in with less force than it slides out. the pressure from fluid helps to seal things up. Worn seals show a 'flat' on one edge/lip only when removed
PJ