brake caliper piston / bore condition

[Bukowski]

I need to judge whether my piston/bore is too far gone. guidance for how much corrosion is too much, how to measure/judge the fit, test the caliper on the bench, etc. appreciated.

one piston I have could be worked out by hand with turning motion, pulling - no air compressor required. is that a sign? there's some corrosion, but also inside the piston - how important is that? (I'll try to get a photo posted for this if there is enough demand).

EDIT: also links to any prior discussions of importance would help too - this kind of thing is not as easy to Google as others.

[Bukowski]

here's the photo

[962porsche]

any rust is to much it will over time ruin the seal . good used calipers are 50 to 75 dollars from bone yards . go to car-parts .com

[Volhv]

While it may look like rust, it could be mostly gummed up all brake fluid.
Take a red scotch-brite pad to it and clean it up. If there is pitting, it's toast. If not, you're good to go.

[Bukowski]

Originally Posted by Volhv

While it may look like rust, it could be mostly gummed up all brake fluid..

right - it looks like that I thought.

what about the rust/corrosion inside the piston - is that worth worrying about? why is it designed like that anyways - keep it light?

Originally Posted by Volhv

Take a red scotch-brite pad to it and clean it up.

would 0000 steel wool be adequate? what about scotch brite gray (ultra fine)?

Originally Posted by Volhv

If there is pitting, it's toast.

is this obvious? what would constitute a minor pit? i.e. is there a good test for this - straightedge? force fluid through the assembled caliper?

[Volhv]

The most important part is the piston. If there is a bit of pitting inside the bore it should be fine, since the piston slides on the stationary seal... that's the only sealing surface.

The piston is hollow for cooling purposes, and it rusts to hell because it's always exposed to the elements. I painted the inside of mine to prevent this.


A bench test of the caliper sounds like it would be difficult to achieve.
If you clean them up, and use new seals they are bound to be better than before. If they still leak, you're out only 8 dollars per caliper for the seals.

Gray scotch-brite should work, not the white stuff though, way too fine.

[Gawernator]

Nice job on the painting

[AaronM]

When I redid the brakes on my motorcycle I polished up the pistons and the inside of the caliper bores with some 600-grit sandpaper. Anything that's suited to polishing will work.

[Bukowski]

Originally Posted by Volhv

The most important part is the piston. If there is a bit of pitting inside the bore it should be fine, since the piston slides on the stationary seal... that's the only sealing surface.

The piston is hollow for cooling purposes, and it rusts to hell because it's always exposed to the elements. I painted the inside of mine to prevent this.

yes that makes sense.

that pic is great - It'd be interesting to see how the paint job held up to the elements - any pics?

[Volhv]

Thanks
Unfortunately I have not installed them yet. I cleaned, painted, and resealed them in the summer. They are a spare set. I hope the paint lasts. I baked them according to the instruction, and everything.

I have a "before" picture, on my blog.
944: Front brake rebuild.

[Bukowski]

Oh, that's you! Cool site! Was reading it before.