'79 850 Rear Caliper Question

[Goose]

The rear caliper on my 850 bagger was dragging, badly. So I pulled it to try and clean it up. I took it apart as far as I could, but can't budge the pistons in either half. I've got the rubber dust seals out, but do I have to remove the piston seals to get the pistons freed up? The dust cover was missing (who knows how long) so I'm not surprised that a little corrosion has gotten in there. I'm afraid to pry on the pistons. Don't want to screw them up any more than they are.

Suggestions?

[Guest]

Some people use air. I just hook up the brake line and pump the lever until one or the other breaks free.

[twr1776]

Make sure to hold your hand over the piston if you use air so the piston does not go flying when it breaks loose.

[bwringer]

Make sure to hold your hand over the piston if you use air so the piston does not go flying when it breaks loose.

Erm... DON'T hold your hand over the piston, especially if you're using compressed air. Use a rag or something -- sometimes when they finally pop loose, they come flying out of there at high velocity. Owie.

[Goose]

Some people use air. I just hook up the brake line and pump the lever until one or the other breaks free.

That's what I was leaning towards doing. So, does the piston seal come out on its own?

[salty_monk]

You can also pump grease into them too....

Dan

[ptm]

That's what I was leaning towards doing. So, does the piston seal come out on its own?

The piston seal is in a groove on the inside of the cylinder so, no, it won't come out until the piston is out. Then it's fairly easy to get the seal out.

[twr1776]

My bad on not mentioning to use a rag to protect your hand if you are using compressed air. Peel the old seals out with a non metal scraper or if that does not work you can use a pick if you are careful not to nick the inside of the bore.

[lurch12_2000]

I assume the calapier is off, then you don't want to reconnect line to try pumping brake. This will require too much time and you may get brake fluid everywhere when the piston pops out.

Definitely use compressed air and a thick rag stuck in between unless you like the feeling you get when using a hammer on your finger tips

I've used solvent and fine emery cloth too clean up piston and inside of caliper. Usually you replace the seals and use air to blow out any dirt after cleaning

I wouldn't use grease?? as this would be hard to clean out after

[Goose]

I assume the calapier is off, then you don't want to reconnect line to try pumping brake. This will require too much time and you may get brake fluid everywhere when the piston pops out.

Definitely use compressed air and a thick rag stuck in between unless you like the feeling you get when using a hammer on your finger tips

I've used solvent and fine emery cloth too clean up piston and inside of caliper. Usually you replace the seals and use air to blow out any dirt after cleaning

I wouldn't use grease?? as this would be hard to clean out after

Good advice all around. Thanks. Not sure how I'm going to use compressed air. I've got a compressor, but don't know how I'll hook it up to the caliper. Might need an adapter, no?

[bwringer]

Good advice all around. Thanks. Not sure how I'm going to use compressed air. I've got a compressor, but don't know how I'll hook it up to the caliper. Might need an adapter, no?

Just stick the nose of the blow gun in the inlet and squeeze the trigger. A little air leakage won't matter at all -- the piston will come flying out at Mach II. If you really want to be fancy, use a blow gun with a flexible tip that will form a little bit better seal with the inlet. But that's not really needed, usually.

[spyug]

In the last 4 years I've rebuilt a half dozen calipers or more and never had any luck with the compressed air method. A grease nipple and gun works well but you have the added problem of having to get all the grease out of the caliper. Grease will contaminate the brake fluid so the caliper needs to be well cleaned.

What works for me is just the normal setup of master cylinder, hose and caliper. I have a spare master cylinder and lever set up on a piece of handle bar which I hold in a bench vice. A Short length of hose to the caliper which sits in used metal pie plate. Fill and bleed the setup as if it were on the bike. Pump the lever and out comes the piston. Has worked everytime so far.

I'm sure you are aware of this but if not make sure you clean all gunge out of the piston bore and examine the piston and bore for nicks and rust. Rust in the bore can be buffed out with emery cloth or wet and dry paper. The piston is hard chromed so it too can be cleaned with very fine steel wool or scotch pad. If there are any pits or gouges in the piston that go deeper than the chrome the piston should be replaced. Rust will form in these pits and it can be enough to freeze the piston in place if the bike sits for anytime like over the winter.

Don't fool around with brakes you never know when you will really need them.

Good luck on the rebuild.
Cheers,Spyug.

[Nessism]

Compressed air has worked for me every time (not to go against spyug). For dual piston calipers like many of the rears, you need to make sure one piston doesn't come out too early or all the internal pressure will be released - hold the first piston back a little to get both pistons out as far as possible.

I concur about inspecting the piston carefully; pitts in the metal are show stoppers, don't even try to reuse the piston or you will get leaks at the point where the pitt passes over the seal - not good.