The front brake on my 2003 Honda XR125L project bike needs rebuilding as it's seized. I find the easiest way to get seized pistons our of a brake caliper is using the same hydraulic force they are designed to use.
First things first, let's give it a bit of a clean. Bilt-Hamber Surfex HD is really good at this sort of stuff since it's a surfactant.
I have a motorbike master cylinder and a short piece of pipe to mount it on so I can get some force on it.
I find the easiest way to fill up an empty master cylinder, brake hose, and caliper is using a brake vacuum pump.
Pumping the lever a few times I can see only one of the pistons is seized. The natural tenancy of a hydraulic system is to find the easiest path to release the pressure. In this case that would mean it would move the working piston. I don't want that I want to move the seized one. So I use an old brake pad to block the movement of the working piston as all the hydraulic force is focussed on moving the seized one.
Once both pistons are free, I can remove the brake pad and push them out.
One piston popped out under hydraulic pressure and the other could be pulled out by hand now it was freed up.
A stainless steel roasting tin makes a good parts tray and contains the mess in one place.
I have a rebuild kit I bought off eBay that includes stainless steel pistons, so they are not going to corrode again in the future. It's more expensive than a mild steel rebuild kit, but still cheaper than OEM. The kit included seals, rubber gaiters, and a new bleed nipple. Sadly the bleed nipple was mild steel, so I will change that for a stainless steel one.
Before I can start putting it all back together, I need to clean out the groove there the seals sit.
I use a pick to remove the old seals, then I scrape out any dirt and most importantly corrosion from the groove. This step is critical - all the problems fitting new seals come from not cleaning the groves out before trying to fit the new seals. The new seals will not sit flat if there's corrosion left in the grooves. Be careful not to scratch the piston bore with the pick whilst you are scraping the grooves clean.
Here I have installed the new hydraulic seals in the bottom groove for each piston. The hydraulic seals and much wider than the thinner dust seals, so will only fit in the bottom groove.
Next were the dust seals. These go in the top groove for each piston. At first only one of the seals would fit and the other didn't fit right. It was bunched up and wasn't lying flat. I removed the seal and noticed there was some oxidation that I had missed, so scraped that out with the pick. When I reinstalled the seal, it fitted much better and would lie flat.
There's a small packet of red rubber grease in the kit. It's next to impossible to install the pistons if you don't use it (and the seals are not laying flat). I only used a small amount, but made sure it was covering the seals.
The new pistons are on the left, the old on the right. The old ones cleaned up mostly ok, so I could have reused them, but I don't want to have the pistons seize again in the future because of corrosion.
The easiest part of the entire process. The new pistons pushed into place by hand with little force needed. If you are struggling make sure the seals are lying flat and that you have applied the rubber grease. This should be a really simple step, if it's not then something is wrong. Don't force it otherwise you'll just tear the seals, they are only rubber.
Almost ready to go back on the bike.
I use some waterproof grease on the slide pins.
The stainless anti-squeal shims get a clean and are reinstalled ready for the pads.
I got a new stainless slide pin for the caliper. The coated mild steel ones always rust in the end.
It's all ready to be put away ready for the rebuild of the rest of the bike.
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