Brake caliper disassembly

I want to replace the pistons in my 850's front brake with stainless versions and I'm confused as to how to get the old ones out. Presently, I have loosened the plug with a proper peg spanner and removed the caliper from the forks. It's still connected to the hose.

This is a functioning brake, so I don't expect anything to be seized. In a perfect world, what would be the procedure for getting the old pistons out?

TIA

Screw the plug back in.
Put a piece of wood, like quarter ply, between the pucks.
Shoot air into the fluid port.
One of the pucks will shoot out. Usually the wrong one, the outside
one. So what I do is with the thin wood or piece of metal clamp the
outer puck to the caliper so it cannot move and the inner has to move.
Remove the strap and clamp and the puck. Unscrew the outboard plug
and push out the piston.
Clean the caliper in alcohol or brake fluid.
Make sure the ring seal grooves are clean, put in the seals, ease
in the inboard piston well lubed with brake fluid.
Do the outboard one. Screw in the plug.

I see that RGM offers pistons with a threaded boss. Looks like a good
idea but Ive never used them.

The only addition I would make to Onder's list is to:
1) Install the inner seal and piston in the normal manner
2) Push in the outer piston past its seal groove (with something between the two so you can push the outer piston back out in step 4)
3) Insert the outer seal
4) Wedge the outer piston back into place.

That way, the rounded side of the outer piston is what slides across the seal, rather than the sharp edge that eventually sits against the brake pad. Thaks to hobot for that one.

Nathan

I also found that it's best to have the pucks just far enough back to get the pads in or they are difficult to bleed. Even put compressed air into the inlet port to push the pads against the disc to make the bleeding process easier.

Doesn't the inboard piston have to come out through the big hole? As in, the outboard one has to come out first?

maylar said:

Doesn't the inboard piston have to come out through the big hole? As in, the outboard one has to come out first?

Click to expand...

Yep; sure does. So, back to your original question of how to get the pistons out. Are you able to get the outer piston out? Usually, it's the inner that's a pain. I used a set of snap-ring pliers to help get a bit of grip to the inner piston. Rigging something to block the fluid passage to the outer piston, then applying air to the brake line port will sometimes push the inner piston out. A lot of guys have had to add heat to the caliper in hopes of having it expand away from the steel piston.
Some details and/or pictures would help with suggestions.

Nathan

My error on disassembly of the pistons! :-(
The idea is to get the inner piston to move. Once it moves it generally can be gotten out.
That is why you want to keep the outer one sealing so air pressure works on the inner one.

Heat helps but heat as in boiling water hot no honk on the torch hot.

Ive seen where a quarter inch hole is drilled from the outside, the piston drifted out
and the hole threaded and a blanking screw installed. Last resort but seems workable.

Be sure to carefully inspect the bores after cleaning. If one of the pistons is hard to get
out it often means it has been cocked or hung for a while and the bore is not true and it
will leak or hang up.

Slide a bolt and nut with the nut screwed onto the bolt until the bolt thread is flush with the nut, make sure the bolt with nut fits inside the piston horizontally with about 1mm clearance. Holding the bolt with a spanner unscrew the nut with another until it is a tight fit inside the piston. This give a good fixture to use a couple of levers i.e. screw drivers etc. to lever the pistons out. This method works well if the compressed air doesn't work due to corrosion or other factors causing stiction.

Mr.Sparks said:

I also found that it's best to have the pucks just far enough back to get the pads in or they are difficult to bleed. Even put compressed air into the inlet port to push the pads against the disc to make the bleeding process easier.

Click to expand...

Easy way to bleed any hydraulic brake system is to obtain a Gunson ‘s G4062 Eezibleed Kit.
To fit M/C obtain a Lucas electric bullet, and use a small dia tube to connect between bleed nipple and kit.
:idea: P>S. DO NOT use more than 24 psi in tyre :!: :!: :shock:

Onder said:

Ive seen where a quarter inch hole is drilled from the outside, the piston drifted out
and the hole threaded and a blanking screw installed. Last resort but seems workable.

Click to expand...

That's what I had to do since the pistons were rusted solidly to the caliper bores. I used a stainless grub screw to fill the tapped hole and sealed it up with epoxy. The original design should have included a smaller, screw in plate on the back side.