Movement of Pistons in Brake Calipers

[PhillipM]

When you clean the bodies also clean out the seal grooves.

This, the groove shape is one of the most important bits of caliper design, and the differences are all in the tapers and small radii - make sure it's spotless.

 

[Ross365]

The man-hours absolutely do not count; I tend to buy cars and just keep them for ever, and do fairly thorough maintenance on them. I have a good and able friend who runs a garage, but it's very rare I go to him for anything.

An awful lot of the calipers on the market are re-builds and I very much suspect that some are pretty good and others much less good. Old calipers are re-cycled and I wonder just how bad they have to be that some combination of shot-blasting and acid dipping doesn't make the look ok for re-use.

 

[Turbo]

Yes the caliper sliding pins, I only use copper on the back of the pads and where the ends sit in the calipers.
Only seem to be able to get silicone off amazon.
I'll order a tube, didn't realise it was just too protect the rubber, thought it might have some magical high heat properties as well. Cheers

Any good motor factors should stock 'red rubber grease', its only a few quid for a big tub. It does the same job as silicon grease but its a lot cheaper.

 

[Bryan Williams]

Any good motor factors should stock 'red rubber grease', its only a few quid for a big tub. It does the same job as silicon grease but its a lot cheaper.

Cheers.

 

[Ross365]

My mistake, a search revealed Celica calipers that were chrome plated and I assumed the OPs calipers were chromed as well as the pistons. (The chromed calipers were aftermarket units.)

Most of the the after-market calipers are supplied with a shiny plated surface of some kind, but whatever the plating is, it goes rusty PDQ.

Both calipers and pistons are like new now (actually, the calipers are re-painted with zinc phosphate primer) and I'm expecting the new seals on Wednesday (complete with both silicone and red greases), at which point I'm confident that rebuilding will be a breeze . Why do some pistons just push in first time, but others just don't want to know?

 

[Fastdruid]

Where I've had issues it has inevitably been either corrosion or that nasty crystalline mess that brake fluid degrades into _behind_ the seals and pushing them too tightly into the pistons.

A scrupulously good clean of those seal grooves has worked wonders, as has using red rubber grease all over them when fitting to reduce the potential for both corrosion and for the brake fluid itself to get behind and then crystallise.

We shall however never mention the "6 pot" Tokico POS calipers that seemed to require such an effort every single sodding year. Fantastic when freshly fettled but went off quicker than week old milk on a hot day (not to mention their tendency to wear pads diagonally and wear gooves into the caliper bodies as they had no wear plates).

 

[slim_boy_fat]

Why do some pistons just push in first time, but others just don't want to know?

Purely to expand your range of knowledge/experience.......

 

[PhillipM]

We shall however never mention the "6 pot" Tokico POS calipers that seemed to require such an effort every single sodding year. Fantastic when freshly fettled but went off quicker than week old milk on a hot day (not to mention their tendency to wear pads diagonally and wear gooves into the caliper bodies as they had no wear plates).

Square cut seal grooves, common on a lot of the bikes around that age, they all did the same thing, no expansion of the seal on piston retraction.

 

[Ross365]

Square cut seal grooves, common on a lot of the bikes around that age, they all did the same thing, no expansion of the seal on piston retraction.

Which leads nicely to the subject of piston retraction, which I never considered much until I stumbled on a youtube vid saying that it was all down to the square section seal, but even that leaves me guessing a little. Any experts?

 

[Al Strachan]

the square section seal has such a grip on the piston and moves with it to a degree (including the oil pressure force), so when your brake pedal is released, the seal naturally retracts to its central position and ever so slightly releases the brake.

 

[Ross365]

the square section seal has such a grip on the piston and moves with it to a degree (including the oil pressure force), so when your brake pedal is released, the seal naturally retracts to its central position and ever so slightly releases the brake.

This is going on at what is almost a microscopic level. I think it's one of those things that if I didn't know that it does work, then I would suspect that it wouldn't work.

 

[PhillipM]

Which leads nicely to the subject of piston retraction, which I never considered much until I stumbled on a youtube vid saying that it was all down to the square section seal, but even that leaves me guessing a little. Any experts?

Not the seal as such but the groove has a taper, varying radii in the bottom to adjust the amount of retraction, etc. The groove is more important than the seal for controlling where and how much retraction there is.

 

[Ross365]

Not the seal as such but the groove has a taper, varying radii in the bottom to adjust the amount of retraction, etc. The groove is more important than the seal for controlling where and how much retraction there is.

The groove needs be very clean, but the methods used in servicing a used item need to be quite "kind"!

 

[Dcal]

Interesting thread and very timely for me, given I need to refurbish the calipers on my alfa.
I have two sets and decided to paint them all while I was at it.
They are alloy and are painted with a very thin coating that has held up well.
They don't look too bad on the outside but the insides, where the pads sit, are a lot worse and badly pitted.
Because of the pitting I decided to split the calipers to get at the pitting and paint these areas properly.

The inside of the bores are painted and generally in good shape despite some of the pistons being badly rusted and stuck.
Most of the pistons came out with compressed air.



Two wouldn't, so I made an adapter for the grease gun which did the job. (Tig brazed a grease nipple to a short length of brake pipe)



I used the number punches to mark each set before splitting them.

Huge difference between the pistons, some OK, some scrap.
I will make an "OK" set out of the existing good ones and buy a new set of pistons for the other set from Big red (£70 for 4, thank you very much)
The calipers have a manufacturing date cast into them, so I'm tempted to keep the set I use original, with the cast iron pistons.


I blasted all the bits yesterday and they are ready for paint.



Now the point of all this rambling is what paint to use on the calipers?

I was going to give the insides a coat of epoxy primer, built them back up with new bolts, another coat of epoxy to the complete caliper then top coats. I don't care if all the nooks and crannies on the insides of the calipers has top coat as the epoxy should give enough protection.
However, of the epoxies I have (Jotun, Selemix and Sherwin Williams) only the Jotun TDS gives a max operating temp of 120 C which is too low.
I could just go with a caliper paint but do 1K products work as well and I never think a one coat system is the right solution.
Hopefully someone can point me in the right direction.

Thanks and sorry for hijacking the thread

 

[Ross365]

I am a huge fan of zinc phosphate primer for all car parts. Good rusting-resistant paint.