Help Rebuilding Front Calipers
 

I’m in the process of rebuilding my front S calipers. I’m trying to reassemble my first one and I can’t get the pistons back in. Each one goes in about half way and stops. I’ve tried using force and one piston will depress slightly and then come back up, the other doesn’t budge. The manual states that the pistons should slide in easily but that’s far from the case for me.

Any suggestions or tips?

Make sure your bleeder valve is in the open position to allow the pressured trapped air to escape. Pre-lube the piston with brake fluid. Good Luck!

I haven’t installed the bleeder valves yet and lubed the piston and new seals with fluid. I actually accidentally tried putting one on before I put the seal in and still had a problem

Are these new Pistons? If you are using the old Pistons really check them for rust flakes, and or rust flakes in the bores within the calipers. Did you blow air throughout the calipers to be sure all the passages are open? Cleanliness is everything here. If you get new Pistons buy the stainless steel ones. They should slide right in even with the new rubber in, but try them without the rubber first , then blow them back out with air.

Without the seals installed, try the lubed piston alone first to make sure no interference by whatever the cause, then install the seals and finish.

Hint: to prevent future piston seized on next pads renewal, fill under the dust boot with synthetic grease at the small end, and at the larger boot end especially under the C-clamp opening gap where water can get in under the C-clamp open gap which does not applies pressure on the boot. With this the exposed piston will not be corroded.

Is this a caliper with the anti-knock back post in the bore? That may be the issue. If so, you might try removing the pistons and cleaning out the hole in the backside of them. Maybe it hydrolocking.
I know they can be a bear to get to push back in compared the the calipers without them.

Some good tips here- PMB - Caliper How To