Stan944

I'm probably boring most of you with these questions on rebuilding brake calipers, but here is the next one:
There is a brown film on the piston (probably similar to what Peckster found in his case, and if I wasn't convinced enough NOT to use anything but brake fluid to lubricate the piston seal, this would do). It can be rubbed off with a nail, but it's tedious. I want to use sand paper to clean it, would grit 600 be fine enough in order to keep the surface smooth enough for a good seal. If not, what else? I could probably use a polishing compound on a cloth afterwards. Brake cleaning fluid didn't remove this film.
Anyways, I'm glad I started this project, because there was so much sludge inside, that it was a matter of time until the piston seazed.
Thanks, Stan

83na944

Stan,

I had a lot of gunk in my right front caliper piston, too. The rest were ok.

I don't think I'd use sand paper on the piston. It has a highly polished surface, and I think it would be hard to get that back. If I couldn't find a solvent that would remove it, I'd just leave it alone. You might try laquer thinner or paint thinner.

BruceWard

I boiled mine. You have to get them extremely dry afterwards. But boiling mine took the crystalized brake fluid off without needing to use anything abrasive.

I also boiled the crossover pipes.

The only similar experience I had to work from is gun cleaning. I would pour boiling water through a gun barrell to clean off the powder residue. So I tried the same with the calipers.

Perhaps it is worth trying.

Stan944

I'll try boiling, thanks. The film is close to where the seal would touch, so I definitely need to remove it.

Scott at Team Harco

I have a 20 gallon parts cleaner filled mid-way with mineral spirits. I soak the pistons in there for a day or two then clean them with "Scoth Brite" pads. You can do the same with spray brake clean. The pads do a good job of scraping off the bad stuff and leaving the good stuff behind.

Stan944

I did the first caliper. Boiling in water didn't work, there was still lots of deposits. I removed them with 600 grit sand paper, polished with grit 1500, and finished with buffing with a high-gloss polishing compound. Grit 600 and 1500 for the caliper bores (didn't touch the sealing groves).

EDITED: I cleaned it after each step to make sure there were no remnants of the last-used sand paper.

I realize that most books say no-no to resurfacing the pistons, but there were scratches before I started. The surface looked shiny (and perfect to naked eye) at the end.

If I did it wrong, and the brakes fail, this might be my last post here, :-)

cheers, Stan

89magic98

I was thinking of using that "0000" steel wool or something to clean mine up when I have them disassembled.

Would a steel wool pad (made of plastic) be safer?

Stan, what kind of polishing compound did you use?

So, this means that doing any kind of honing is definitely a bad idea?

Jon Moeller

I used scotch-brite pads for the pistons on my BMW. I think that it's a much safer solution than sandpaper, but I doubt that fine grit sandpaper would do any damage.

-Jon

Stan944

I don't know about steel wool, check also the post by Scott at Team Harco.
My guess is that if the final surface looks shiny and perfect to the naked eye, than it sould be good enough. You can test your polishing method on the piston bore, as it dosn't need to be leak proof. Anyway, I had visible scratches on the piston (at the sealing area), and the brakes looked dry upon dissassembly (i.e. didn't leak fluid).

I'll check the brand name of the polishing compound, and post it tomorrow.

I think you can safely hone the piston bores (with the exception of the sealing grove), as they do not provide sealing surfaces.

EDITED:
I used Mibro (PN 698091) high gloss polishing compound #6