I'm cobbling together a working brake from all the bits and in the process of marrying a nos no8 M4 1/2 caliper to a used 1/2 that had very sticky pistons.
Usual nightmare of freeing them made a bit worse by the other half being new and any pressure and they're itching to jump out, so you have to prevent them from doing so which is always where the nightmare is, especially when they're stuck in tight.
I think ive found an easier way of isolating one side from the other and its simple. Just a piece of steel to lie along the slot where the rotor runs, so one side you dont want to come right out you put the pads in, and the other side you leave open, for the pistons to pop out as you ramp up the pressure.
If one moves and the other doesnt, then add some shim to the free'd side to stop it moving out further, which puts all the pressure on the last remaining piston. If you see what I mean.
Also found you might have to change the thickness of the shims to to side/the other side them out.
So pistons out, old seals out and a clean up of the recess for the seals using a pin and thinners. 1600 W&D on the piston recess leading edge, just a tickle to smooth any corroded edge that might be pressing in.
Rest is just reassembly, and fingers crossed its not a dot 5.1 waterfall.
These are mini m4's, which we all know started off with a steel piston, with they then changed to the phenolic pistons they did as a replacement, and are still using today on their latest brakes.
What was the problem with the piston that made it stick in some(Ive old battered minis working flawlessly) ?.
My thoughts are its heat, especially in the M4's with the smaller pistons. In that I think it is deforming them, and the plastic ones cant be, hence the continuing to be used, which is about 18 years down the line. So seems phenolic is the material.
The two steel pistons I took out, apart from a bit of scuffing along the top, from that edge being mostly exposed, are still very very smooth, and the seats are also very clean with the smaller looking pristine, and it was just as difficult to remove. So must be something different about them, and I reckon this is deformity from heat :?
Can anyone confirm this ?.
I wondered if the heat, causing the expansion of the metal itself, didnt return to the original diameter, so it was left slightly bigger and just jams in ? Just speculation. It sits against a seal, which will be a tiny bit proud anyway especially if its designed to expand a bit, and whatever change has happened to the piton,its quite extreme.
I suppose i'm going to find out in the next hour if it's the piston/seal or actual seat
But if anyone has any thoughts on this , love to hear them. Because it has to be something thats changing.
Just wondered lot of geeks here
A pic for folk to look at always better with a pic to look at
Usual nightmare of freeing them made a bit worse by the other half being new and any pressure and they're itching to jump out, so you have to prevent them from doing so which is always where the nightmare is, especially when they're stuck in tight.
I think ive found an easier way of isolating one side from the other and its simple. Just a piece of steel to lie along the slot where the rotor runs, so one side you dont want to come right out you put the pads in, and the other side you leave open, for the pistons to pop out as you ramp up the pressure.
If one moves and the other doesnt, then add some shim to the free'd side to stop it moving out further, which puts all the pressure on the last remaining piston. If you see what I mean.
Also found you might have to change the thickness of the shims to to side/the other side them out.
So pistons out, old seals out and a clean up of the recess for the seals using a pin and thinners. 1600 W&D on the piston recess leading edge, just a tickle to smooth any corroded edge that might be pressing in.
Rest is just reassembly, and fingers crossed its not a dot 5.1 waterfall.
These are mini m4's, which we all know started off with a steel piston, with they then changed to the phenolic pistons they did as a replacement, and are still using today on their latest brakes.
What was the problem with the piston that made it stick in some(Ive old battered minis working flawlessly) ?.
My thoughts are its heat, especially in the M4's with the smaller pistons. In that I think it is deforming them, and the plastic ones cant be, hence the continuing to be used, which is about 18 years down the line. So seems phenolic is the material.
The two steel pistons I took out, apart from a bit of scuffing along the top, from that edge being mostly exposed, are still very very smooth, and the seats are also very clean with the smaller looking pristine, and it was just as difficult to remove. So must be something different about them, and I reckon this is deformity from heat :?
Can anyone confirm this ?.
I wondered if the heat, causing the expansion of the metal itself, didnt return to the original diameter, so it was left slightly bigger and just jams in ? Just speculation. It sits against a seal, which will be a tiny bit proud anyway especially if its designed to expand a bit, and whatever change has happened to the piton,its quite extreme.
I suppose i'm going to find out in the next hour if it's the piston/seal or actual seat
But if anyone has any thoughts on this , love to hear them. Because it has to be something thats changing.
Just wondered
lot of geeks here A pic for folk to look at
always better with a pic to look at 
