front brakes gradually seize, faster in warm weather

[Guest]

Greetings everyone, and thanks in advance for any suggestions. I've entirely rebuilt both calipers and the master cylinder on my front brakes (198 GS850G) and to my amazement, the brakes seize up on their own, even when the bike is not being ridden. It happens more frequently during the hot weather than in the cold, and it happens regardless of riding time. I end up bleeding half a teaspoon out of one caliper or the other every time, and it works fine again for some amount of time, then it seizes again. I have obviously changed the fluid several times, always using DOT3-4, but the same thing has been repeating itself for a couple of years now. Ideas??? Many thanks and blessed rides to all in Nature's pristine beauty!

[eil]

The most likely cause is not cleaning the calipers and master cylinders quite well enough when you rebuilt them. Everything has to be pretty spotless inside for the brake system to work properly. In particular, the piston in the MC has a teeny-tiny hole in it that relieves pressure in the system when you let off the handle. When this is blocked by decades-old crusty brake fluid (and it very often is), the brakes tend to seize up. Sometimes while riding. Not a good thing!

[Steve]

Eil has given the most-likely culprit, and I agree with him.

Hard to tell what year your bike is, as there is a digit missing, but at the bottom of the master cylinder reservoir are two holes. One is larger (about 1.5-2.0mm?) that is usually rather visible. There is another one that is closer to the output fitting that might be under a small plastic tab on the reservoir that might be about 0.5mm in diameter. If it is clogged, pressure can not equalize from the calipers as they warm up, and the resulting pressure increase will just apply the pistons. Of course, that just builds more heat (and pressure) which has nowhere to go, so it applies more piston, etc.

The proper fix is to clean that port. You might be able to open up the master cylinder and blow the port open with your compressor, but it might be necessary to use a strand of wire to poke through the crud. Either way, be sure to flush it well, to get rid of the crud.

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[Highway_Glider]

Hi AdventuersUnbounded.

Welcome to the forum.

I am no expert. Even less so than the previous members But I agree. The master cylinder is the more obvious culprit.

It sounds like the fluid isn't returning back to the reservoir.

It could go like this:

At night the fluids contract and suck a very small amout out the reservoir. In the day time or while riding the fluid warms up and expand. The piston move toward the disk, never to return. At night the piston has just enough resistance that when the fluid cools and shrink a little more of it is sucked out of the reservoir.

I would start with the master cylinder. As said there is a "Pinhole" in it. It must be clean.....And stay that way. In doubt change the fluid.

When you rebuilt the system did you flush the lines? Could garage grime get in there while other parts were elsewhere?

[chuck hahn]

The hole isnt in the piston..its in the housings bore right at the very bottom of the resevoir well. Wire off a wire brush usually clears then hole nicely.

Second thing that causes failure is the groove the piston seal set in in the caliper body. Crud builds up behind it which naturally causes something to be displaced. the thing that gets displaced is the seal. it gets tighter and tighter around the piston and eventually the piston will lock up..and usually in the outer brakes applied position.

Lesson being..clean everything meticulously.