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Restoring brake calipers

what measurements or tools are required when judging the quality of the surfaces of brake caliper pistons and bores? what about bench-testing?

I know I can buy new if in doubt, please do not answer as such.

I asked about plating on calipers the other day here, and most of the pertinent info is there too:

in short, ~25-year-old Ate single piston calipers from a late-model year Porsche 944.

If, after a very light honing, the bore is smooth and, more importantly the seal grooves are clean and correct, and the passages are clear, you’re good to go. The bores aren’t the critical part, because the seal is between the o-ring in the caliper and the smooth sides of the piston. Is that your caliper’s design?

The pistons must be pristine.

If you see a lot of rust in the area that the piston seal will be riding, then buying new would be the better option. Rust in the bottom of the cup is not a problem, just remove it. If you can wipe the sides of the bore with a couple swipes red scotchbrite and it cleans up, they are good.

The piston should be cleaned up too, but the only area of real concern is in the seal groove, you don’t want any pitting in there.

@keith - “The piston should be cleaned up too, but the only area of real concern is in the seal groove, you don’t want any pitting in there.”

Most pistons I’ve seen don’t have a seal groove, that’s in the caliper. The pistons do have a dust cover groove.

Have you seen pistons with both grooves?

my bad, don’t know what I was thinking. I have rebuilt calipers dating back to 1968 so I should have known better. Not as a job but on my own vehicles, last time was about two years ago. I think I got caught up in some of the other’s posts about how important the bore of the caliper is and honing the caliper and all that and my head just went in that direction. Unincumbered by the thought process.

I guess my advice now is that the caliper bore isn’t critical, just the groove that the seal sets in. The piston walls need to be pit free. Most pistons are a high chrome content stainless steel as I recall so they should not have any rust on them.

edit: most of the posts that led me astray were in his other thread about the plating.

I think folks are so used to the importance of honing master cylinders and drum brake cylinders they apply the same logic to disc brake calipers, when the caliper bore actually doesn’t do the sealing, it’s the o-ring, the caliper groove, and the piston.

great comments, thanks.

piston walls need to be pit free.

by eye? or is there some sort of straightedge, a dial gauge, or some such tool?

most of the posts that led me astray were in his other thread

something I said?

should I start yet another thread about using cylinder hones (after googling the subject, of course)? Also someone in the other thread mentioned that cylinder hone tools apply to brake piston cylinders - I was not aware. it always sounded like it was just for engines.

is there a way to quote specific users automatically instead of by hand, like this:

Someone_In_Particular wrote:

some sort of comment by Someone_In_Particular

Don’t start another thread, it just gets confusing. Ask your questions here.

right, so cylinder hones - easy to find, but I am learning from square one.

Again, these are a spare set of calipers so if I mess them up there’s no real harm done - especially if I can bench test them adequately - i.e. consider these not to be used on my car immediately - only for learning purposes on the bench.

do I need a specific size hone for this caliper? looks like they run on a drill - yeah? is there a “grit” equivalent to these? will I need a specific type of hone? how long do I run it in there for? the piston - looks like the most I can do with that is run light-grit sandpaper/scotch pads on it or it will get messed up.

also - doesn’t rehoning mean there’s more of a gap between the piston and bore, and can lead to leaks out of the seal easier?

I haven’t rebuilt a brake caliper in years. I usually just buy a rebuilt caliper. Last time I tried to rebuild one…I couldn’t find a brake caliper kit for that brake…but I could find a rebuilt brake caliper.


I haven't rebuilt a brake caliper in years.

I would make the distinction between restoration and rebuilding.

that is to say, I consider rebuilding to be getting a seal kit and putting them on greasing the sliding surfaces and getting on with life, while restoration is that plus … well, … rehoning, cleaning, - the whole 9 yards…

but again, maybe it is stupid to restore brake calipers!

‘rebuilding’ should include anything necessary to restore 100% functionality, including honing.

‘restore’ includes ‘rebuild’ + cosmetic stuff.

while restoration is that plus ... well, ... rehoning, cleaning, - the whole 9 yards. -

I NEVER did a restoration “as you call it”. I’ve only rebuilt calipers. I still have all the tools.

Honing is for the wheel cylinders of drum brakes and master cylinders. Just clean out the cavity of the calipers. The pistons should be pit free, visually. Put the seal in and lubricate it and the walls of the piston with brake fluid, not grease.

The tricky part is the dust seal. Put it in the top groove of the cavity, then stretch it over the bottom of the piston without it popping out of the groove. Once you accomplish that, start inserting the piston into the cavity slowly. As the piston goes in, slide the top part of the dust seal up the sides of the piston.

When the piston is in about half way or so, you can slide the dust boot up the side of the piston the the groove at the top. Don’t stretch it or it might pop out of the groove in the cavity and then you have to start over.

Don’t forget to clean and lubricate the caliper pins (bushings). If the pins (bushings) don’t slide freely, the calipers won’t float. The pins (bushings) need new dust seals as well.

That original thread appears to be not found anymore so @texases couldn’t see your comment. Did I miss the mark?

That original thread appears to be not found anymore

well I’ll be - vanished, it is.

I had written at the end of that one too, something that directed traffic over to this thread if it was not about “original plating”, the title of that thread…

here’s the Google cache of that thread:

none of the comments though.

any moderators here?

I’ve seen some threads and some of my post oddly disappear in recent months. I’m not sure if it’s from a technical glitch or over zealous moderating.

None of the threads or posts were controversial before they disappeared.

@JuniorMint - what questions do you still have about the calipers?

texases wrote: "what questions do you still have about the calipers?"

why do you ask?

I would have to say none immediately. does that mean I can’t post back with details in this thread after I do some work on them?

Oh, not at all. I just wasn’t clear if we had addressed your questions.