Hope Piston removal

dyna-ti

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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 :LOL:

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 :LOL:

A pic for folk to look at ;) always better with a pic to look at :D
 

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My_Teenage_Self

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Could it just be the seals dried out a bit and stuck to the piston?

I doubt that you'd heat steel enough to affect it long term, although I appreciate that it'll certainly be affected during use.

also, please do use the hunters silicone lube recommended by hope; it's excellent.
 

Timoth27

Senior Retro Guru
Re:

I’m not sure it’s deformation as such (or maybe as well as) but more like expansion, when the steel heats up it expands just enough to cause stiction. Where as phenolic is a really stable material and as such doesn’t expand when it heats up. Well that’s my theory anyway.
 

mattr

Old School Grand Master
IIRC is was a combination of the steel distorting during heating and also transferring too much heat to the oil, so especially an issue for serial brake draggers or heavier users.
 

mdvineng

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Heavy serial brake draggers can and do soften the Phenolic pistons into a distorted mess and great fun to remove with non splitting calipers. Repaired a few for people with various success rates.
 

dyna-ti

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My_Teenage_Self":1vfmyjcr said:
Could it just be the seals dried out a bit and stuck to the piston?

I doubt that you'd heat steel enough to affect it long term, although I appreciate that it'll certainly be affected during use.

also, please do use the hunters silicone lube recommended by hope; it's excellent.

Hadn't thought the seals might be dry and stuck to the piston as it was full of fluid. I also changed a bleed nipple on this brake as I found that clogged tight. I was sure that was the prob, but not so :( Anyway, could benefit from new bits,for all they cost really.

When i did pump it out, i did a thinners clean and lube, but they werent for going back in easily, which made me think of distortion. You got them out, ffs dont put them back in again :LOL:

Typically by the time I read this all lbs's are shut and I either order online or wait till tomorrow see if the local has it.
I also think given the mount side is new, I'll route the hose into the replaced piston side, so the fluid fills those chambers first, or hopefully will provide more pressure that side :? I've no idea if thats going to do anything, but again cant hurt*




* Step forth 1/2 dozen replies on that :LOL:

I'll also do a pic of using a metal bar to brace one side or the other against to isolate 1,2 or 3 pistons. Of the you tube vids, most it appears are struggling to hold pistons in, while trying to get out a single jammed one.

Timoth27":1vfmyjcr said:
I’m not sure it’s deformation as such (or maybe as well as) but more like expansion, when the steel heats up it expands just enough to cause stiction. Where as phenolic is a really stable material and as such doesn’t expand when it heats up. Well that’s my theory anyway.

Aye, I see what you mean, but this brake is cold. Were the expansion the entire problem, upon cooling it would free itself up. Plus there will be a working temp in the design model. I've no idea personally what brakes get up to temp wise, but the M4's were always plugged as the DH or whatever extreme use, so heat shouldnt be a factor.
 

Timoth27

Senior Retro Guru
mdvineng":2sbtkyzd said:
Heavy serial brake draggers can and do soften the Phenolic pistons into a distorted mess and great fun to remove with non splitting calipers. Repaired a few for people with various success rates.

Obviously there are always exceptions and nothing is Indistrictable but if you compared the reported piston failures to brakes sold I would guess it’s a pretty small % of failure. It’s quite difficult to design/produce a product that allows for miss use/idiots.
 

mdvineng

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Timoth27":2h2cxr44 said:
mdvineng":2h2cxr44 said:
Heavy serial brake draggers can and do soften the Phenolic pistons into a distorted mess and great fun to remove with non splitting calipers. Repaired a few for people with various success rates.

Obviously there are always exceptions and nothing is Indistrictable but if you compared the reported piston failures to brakes sold I would guess it’s a pretty small % of failure. It’s quite difficult to design/produce a product that allows for miss use/idiots.

Do you remember the lightweight discs from years ago, a sandwich of steel\aluminium centre\steel, had a few of those brought to me that heated up enough to melt the aluminium centre and caused catastrophic failure :roll:
 

mattr

Old School Grand Master
Shimano icetech, weigh pretty much the same as a steel disc, aluminium sandwich construction. Still widely available.
 

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