Fluid pressure makes them push out but what causes the pistons to go back in?

Whatever it is, why does it NOT cause the pistons to retract fully every time?

EXAMPLE: Brake pads were spaced far enough apart for my old, worn rotors. The new rotors drag because the pistons did not go back in all the way.

I have removed the pads and easily pushed the pistons back in with my fingers. I imagine the pads will not drag any more (I hope). I imagine they will go back in far enough for the new rotors now that I pushed them(again, hoping). Why did the pistons not go back all the way into the caliper housing on their own?

* Last updated by: Rook on 5/5/2013 @ 2:30 PM *

the cylinder never goes all the way back into the caliper, think about it rook when you change your pads and you push that cylinder all the way in for the new pads, whats the first thing you do before heading out on the road? you pumps the brakes until you get lever back. so you apply brake lever which pushes fluid into the caliper thus pushing the pads against the rotors release lever you release pressure against the pads. if you think about it the pads are alway making slight contact with the rotors, you did what you needed to to know the calipers are working correctly by depressing the cylinder back in to ensure its working correctly. Still my main concern is you trying to fit used pads (which are wore to the old rotors) to new rotors. You need to get your new pads on there before you spend too much time worrying about something that may not be a problem. thats my .02 cents lol

if you think about it the pads are alway making slight contact with the rotors,

That is the answer, hey? It is just that the pads let go of the tight grip on the rotor but they pretty much just stay a breath away after pressure is released. The only thing that will push them away is maybe some flexion in the rotor or something like that. Otherwise they really do drag over the rotor ever so lightly.

Still my main concern is you trying to fit used pads (which are wore to the old rotors) to new rotors. You need to get your new pads on there before you spend too much time worrying about something that may not be a problem

Seems like problem was not a problem at all. The pistons just didn't want to go in after a couple years of sitting out. A little resistance when I first pressed my fingers on them and then they pushed right in. I think if I just would have ridden the bike, they woulda went in on their own. That's the way they are supposed to work or so it would seem.

Yes, maybe I'll get new pads. It is not very hard to change pads if I am going to be switching wheels, might as well have dedicated pads.

I think I may have a warped rotor though. It actually tested "warped out of spec" in my runnout test I did with a dial gauge. Now I hear it touch lightly in the same spot every time it passes through the pads. If it is in fact warped but the warp does not impact high speed performance, I will prolly live with it. If I can feel it in the steering or brake lever, it's gotta go NOW.

I'll address my warped rotor concerns in the Carrazzeria thread soon. Thanks for answering the question about piston retraction, scotty.

CARRAZZERIA THREAD

* Last updated by: Rook on 5/5/2013 @ 9:02 PM *

Master cylinder:

1. Hole to fill the fluid.
2. Hole to bleed the master.
3. Hole to feed thru the lines.
4. Hole for the piston to push past [holes 5-6] and return home.
5. Hole to add fluid to the lines when pads wear = The tiny hole.
6. Hole to return fluid and zero out the pressure back to 14.7 psi = A little larger hole.

Caliper housing:

a. Housing holds a square made ring.
b. Housing holds a piston.
c. Housing has a square groove for the square or quad-ring as opposed to an o-ring.
d. Housing has a hole to feed fluid behind piston and pushes piston in one direction only. Why? You have no pump once you release the lever. The fluid returns to zero pressure.

Reversengineering:

When I press MC piston in one direction, I have to cover the tiny hole so I am now sealed. When open, it has to feed fluid or keep the fluid there ready for a stroke. So it feeds from the reservoir. The little larger hole pushes fluid back into the reservoir it came from. When the stroke happens, there would be vacuum behind the piston if there wasn't that little smaller hole pulling a neutral pull of fluid to equalize 14.7 being drawn from behind.

When I release pressure, the fluid built up behind the piston has to be pushed back out, so this sends fluid back out the little larger hole. Notice how I made no reverse vacuum but to have that fluid return to 14.7. I first had to push the liquid and that is more instant than thinking there is an air cushion between lever and liquid. So have we established we have zero fluid pulling back on the piston but moving back to zero only? No fluid could help pulling piston back.

Quad-Ring:

Rubber has memory. This ring sits square in this machined groove's OD. The piston sit inside the ring's ID. The piston has enough slide so as not to cock to the side, some squid sands the machined finish down. That does not stick the piston. Take your finger and stick it thru you making a round hole with your other index and thumb. Your round hole is the quad-ring. Your finger is the piston. Push your finger until it moves some, then stop. Now, move your finger in and out without moving your finger from your circled fingers.

Feel that give? That would be like how the quad-ring moves the piston back to square. That's where memory is moving the piston, because there is not wear you can see if we stopped the bike a few times. It takes thousands of miles to move the pistons out. It takes every time the piston is pushed, so does the quad move back to square. So this movement is more what pulls back the pucks or more there is zero puck pressure. There is no puck pulling back. There is just puck pressure released and with the memory pulling back the piston, your slight warp that comes around and pushes this one side inward is more disc release [if] not fighting that memory coming back into the disc again.

Disc Drag:

Thru time/neglect, contaminated fluid builds up under the quad-ring. This buildup under the quad will set piston from moving the piston back. So the piston is more set at this position. No rebleeding may help. What might clean around the quad will not clean out what is under the quad. Thus the rebuild, the new quad seals, the cleaning of the groove of all contaminants. No marring of finishes when rebuilding.

...so the seal, the quad ring pulls them back. Given the little "pull my finger" exercise you described, I would think the quad rings just pull the pistons back a tiny bit. Probably just the release of pressure and a little rotor movement would nudge the pistond back in enough so the pads were just skimming the rotors. I will keep the quad ring replacement in mind in case I continue to have trouble with the pads dragging after I have pushed the pistons in by hand. I change brake fluid pretty often but I have never taken the calipers apart.

The seal is out and out flat on 4 sides. It is sort of narrow so the rubber moves very little like you said. So as to exaggerate the rubber wrinkle, the rubber sort of distorts to a push / angle. As the fluid relaxes back to atmospheric pressure, the strong rubber's memory moves back to a straight up | square again. The piston stays in the ring's center. It is being pushed back by whatever [disc] warp there is, plus the memory, plus for every action there is and opposite and equal, so that sort of pulls the piston back and away and I doubt the quad moves back equally in \ direction and then | centers back up? If it does, it's ever so slight so back to center it moves once again.

However, water is heavier than oil. So this will fall under the quad ring, heat, form crystals and grow under that rubber. So when there was fresh oil to displace that groove, now the lowest part is displaced with a tiny bit of water. The tolerance is so exact, any added pressure like a growing crystal or how nature is going to eat that metal to dust, we now stick the piston from moving is one. The other is the quad is no longer square in the groove but under pressure. It's as if someone shimmed a rocking table. It had less friction moving the table out of the way sliding on 3 legs. Now all 4 legs tend to make pushing the table a bit harder to move is that friction drag [back to square] meaning.

I saw some YouTube instructions, including Dave Moss recommend that you open the calipers bleed port before pushing the pistons back in, so that any older potentially contaminated fluid doesn't get pushed up to the MC.
Given Hubs explanation, that does seem like good advice.

huh-huh...too late. I change fluid pretty often though. I'll be doing a complete fluid change soon. Thanks for the tip, oz.

If you want to keep going, it's all about X to Y and = Z Retraction.

A. This is Fork to fork to axle. You are handcuffed to the bottom taper of the fork leg in the lower tree. It's so close how little range you have, the point is in the generic setup from fork to fork: stake a fork where you want it and that is base, home, central. The other fork floats. That means your 3rd variable is the axle float so there is no cocking of the disc angle. X is home Y is fork float = Z is the axle flies thru the [fork's] holes like a bullet thru a barrel.

B. This is Axle to fork to fork. We know the axle will stop in the collar in the one fork is X. The axle has the same O/D as the collar and that is Y. So the deal here is to first tighten the axle into the collar, then pinch the fork bolts down on the collar. The floater fork is going to center itself as you spin axle back out of the floater fork. With wheel in place, spin the wheel and slam the brakes on. X is the locked forked collar w/pinch. Y is the fork float = Z leg is going to float to center with the brakes floating on their pins, the leg floating on the axle. So now your fork is not cocked, but is centered, without tweaking the one fork like this / and the other isl like this |. Fork axle to fork wise, / | is spin and brake, centers the floater fork like this || and you do not need to push the forks down as if that will pop it to center too. You twist back and forth as you push down, but I find the calipers will move whatever needs to be moved from one side to the next? So if the leg is \ | way, the calipers will still call it as if you needed to push down on the forks with wheel.

C. This is caliper to fork to disc. We want everything in square to each other. This last part is caliper pinch to disc spin. X is the pad squared. Y is the caliper. So this deal is to float where the caliper bolt holes being sloppy, we want a hand tight caliper bolt so when you spin, the caliper is X to Y as close as you can float [the pucks square] to the disc. So where the spin begins, the brakes stopping where the clamping squares everything in proportion: It somehow moved the caliper to center(?) in its own home or float... X is the caliper. Y is the hanger bolt = Z lines up as sloppy is what sloppy production does is close enough is the front is end ready to go, sans clean quad-seals and the machined recess it fits in.

* Last updated by: Hub on 5/13/2013 @ 10:22 PM *