Rear brake caliper seizing

[tadakan]

Bit of a long post, but I could really use some help and I'm trying to lay out everything I've done or tried so far.

I'm having trouble getting my rear brake caliper to function properly on my 1980 gs550L. It's a twin piston caliper with one piston on each side of the rotor. Three times now I've been riding along for a while and the rear brake grabs harder and harder until it stalls the bike. The I'm stuck dragging it off the road and pulling the rear caliper off so that I can get the pads out and ride the rest of the way home with just the front brake.

I've cleaned out the calipers pretty thoroughly each time with brake cleaner and compressed air. The second time I replaced all of the seals in the caliper even though they looked fine and were new maybe 150miles/12months ago (i've been working on other stuff so it hasn't been ridden much until recently.) I used a new set of oem pads as well.

The brake line is a stainless line that I made from Earl's braided stainless hose and compression fittings, it's a little bit too long, but it certainly isn't kinking at all.

Tonight I pulled the master cylinder off also and blew it and the caliper out with compressed air and used brake cleaner on the caliper bodies. I'm going to put it all back together tomorrow after I take a look at the pads. I've got another new set if I have to use them, although I'd rather not...

I rebuilt the MC 12 months or so ago with all new OEM rubber.

The only thing I can really see being a problem at this point is a tiny bit of rust on the outside of the caliper pistons.






Do you think that there's enough pitting that that's causing the problem, or do think there's something else going on that I need to deal with?

It would be really nice to not have to worry about winding up on the side of the road in the rain because my brakes seized up...

[Steve]

The problem is not in the caliper, it's in the master cylinder.

On the side of the m/c, there is a large fitting with a hose that goes to the reservoir. Inside that area, toward the bottom, is a hole that feeds fluid to the system when necessary. Just above it, near the top of that area, is a much smaller hole. THAT hole is plugged.

.

[mmattockx]

I think your master cylinder isn't operating properly. If the bleed hole is blocked it will act just as you describe.

EDIT - I see Steve was typing while I was drinking bourbon and beat me by a minute...


Mark

[tadakan]

Thanks for the quick replies guys. I'm pretty confident that that hole is clear now since I specifically blew compressed air through it tonight, but I guess it could've been clogged prior to that. I'll reassemble everything again and hope for the best.

[cowboyup3371]

Run a single strand of wire through that hole before you button it up just to be certain.

[tadakan]

I'd heard of using a single strand of wire to clear the return port on the MC. I wasn't able to find a wire that was thin enough to fit the port and rigid enough that it seemed to be accomplishing anything. The compressed air seems like it's done the trick though (120psi from my compressor).

I rode the bike quite a bit this week and didn't have any more trouble with the rear caliper.

Thanks everyone for your help!

[cowboyup3371]

For future use, take some 18-gauge wire and strip off the insulation a fair amount. Then remove a single strand from it and you'll be able to use that as a poker

[Gorminrider]

a single strand from a sailing dinghy's SS rigging wire is good..it's wavy but works well....or a guitar,piano,mandolin? whichever has the tiny gauge...these are all stiffer than copper.

Though, it might help if you heat the copper red hot and let it cool slowly ...this will harden it (opposite process to steel)