The piston shouldn't be tight, only the o-ring.
1. My piston moves in and then the oring stops it from sliding in the rest of the way.
2. My piston won't move at all as soon as I install the piston I am not close to the ring.
Which is it?
3. What does the inside of piston oring and groove look like as they both meet that groove with part of the oring stuck in there? White crusty stuff?
a. White crusty stuff is water mixed with oil. Not flushed enough every 2yrs say.
b. White crusty stuff expands and then when you hear the brake disc drag, the resistance is more a locking device as the oring is squeezed tighter around the OD of the master's bore/caliper's bore.
c. See why you have to remove the crap under the oring and clean the groove so the oring now can lay flat? If not, the piston slides thru a straight up oring; the piston does not return being the oring never wrinkled forward; the piston now stops where the pressure placed it; the brake pad now drags; the high spot in the disc pushed the pad back, but still drags on the straight running disc. This is this example of a dirty groove, adding the crust in between the inner quad-ring. The same drag in the master is that dirty groove too.
4. Are we taking about a quad-ring? This is how a quad-ring works.
|| --- This is the oil push but right now we are at zero pressure at the q-ring.
\\ <<< This is oil pressure and inside that quad ring is the piston moving.
|| >>> This is oil returning back to neutral as in no pressure. This is called, 'memory.' The quad is so thick it wrinkles in that // forward position and has the ability to square back into that || straight up position. However, the groove and ring have to be clean for that groove and that memory is the cause of that effect. This is how the piston is pulled away from the brake pad. This is how the brake pad is released off the disc. And this [memory] needs to fall back into a clean groove.
It can't with the white chemical reaction under the o-ring/quad-ring. The groove has to be spotless. The rubber material has to be spotless. Piston always moves thru the quad as the pad wears. See it move? Wrap your index and thumb around an empty toilet roll. Push the roll thru some. See your skin moves with the roll, but gives at the cylinder roll you let go: your skin memory lets it sit back in the static? It's that simple.
Quad + Crust + Filthy = No piston move.
Signed,