GS550 Sudden stuck brake caliper during ride

[Guest]

So, I've been working on a 78 GS550. I've got it running fairly well, and took it out for a longish ride on the freeway today (I was commuting, my main bike is out of commission). I made it about 20 miles at freeway speed, before I had some trouble getting up a hill. I pulled over to check things out, and saw smoke coming from the front brake caliper. The disc was extremely hot. The brake lever was completely stiff, and I couldn't roll the bike forward.

I didn't have a wrench or socket on me to crack the bleeder, so I called for a tow. While I was waiting, as the bike cooled off, the brake released itself and behaved normally.

A few days ago I replaced the front brake pad, and bled the brake system and felt like the brake was performing much better than before.

I'm assuming that I'm going to need to rebuild the master cylinder, and possibly the caliper. I just don't get why this would suddenly happen while riding when I wasn't using the brake.

[Steve]

Your likely culprit is the equalizer hole in the bottom of the master cylinder reservoir.

This is a tiny little hole that sits just ahead of the piston, that allows fluid to flow freely from reservoir to hoses. The first couple of milimeters of piston travel will cover the hole, allowing pressure to build to stop the bike. It helps to know that, even when everything is PERFECT, the pads will drag slightly on the disk. There is a bit of friction there, which means there is a bit of heat. Normally, it's not much heat, but enough to expand the brake fluid slightly. When the equalizer hole is plugged, the fluid can not expand into the reservoir. The only other thing it can do is to push on the back of the caliper piston. Of course, that applies the brakes a little harder, which makes more heat. More heat is more fluid expansion. More expansion means more push on the caliper, etc. Eventually, it's pushing hard enough to stop the bike.

A full rebuild of the master cylinder is seldom a bad thing, but you will probably get by if you just clean out that hole. Please note that this is a VERY tiny hole. The smallest drill in a normal set is 1/16", which is about double or triple the size of the hole, so poke carefully. Some have reported success with guitar strings and other such stuff, so find something small, but rigid, so you can poke the crud out of the hole.

.

[Big T]

It's best done by taking the master cylinder off as you replace the front brake line.
Flush the MC well, and also the caliper

[Isoparmesan]

I see, so since I just put on new pads, there was increased drag, increased heat, which caused the fluid to expand and engage the brake. and when it cooled off it went back to normal. makes sense.

good thing I play guitar and have lots of guitar strings. I'll get on clearing out that hole.

[cowboyup3371]

You'll do well to replace the brake lines as well as clean the calipers properly with new seals

[fastbysuzuki]

Sounds like the brake pads are rubbing on the disc and not releasing correctly and generating heat either incorrect pads
(unlikely) Pistons in calliper sticking, or partially blocked return hole in master cylinder. Or a combination of the last two.

[chuck hahn]

I use a wire plucked from a wire brush to poke holes in master cylinders and carb jets.

[Corvette95]

I posted a similar thread about a similar problem with a later 550ES. Did not know about this holes, is it on all three calipers and can you post a picture of what I am looking for or explain access and location of this hole and again, is it all THREE calipers? I am having harder brake effort lately.

[steveb922]

Not calipers - master cylinder.

Having just read your other post I would recommend rebuilding your master cylinders and calipers, and especially replacing your brake lines. They were intended to be renewed after two or three years, not 33.

[salty_monk]

There is a surprising amount of junk in stock old brake systems if you open them up and clean them....

[Isoparmesan]

So, I cleaned the master cylinder and it’s still happening. There were fewer components to clean than I expected. All the rubber parts were still soft and pliable. In terms of the return hole, does anybody have an image of it or a clear description of where it is? There were four small holes on the piston, and I cleared those out. They were very gunked up.

Do the lines get clogged? I haven’t replaced them yet, but certainly can. Are there any recommendations on which ones to get? I’ve never bought brake lines.

[Steve]

In terms of the return hole, does anybody have an image of it or a clear description of where it is?

I don't have a picture, but a description might suffice.

Look in the bottom of the master cylinder reservoir (after draining the fluid). You should see a couple of 'holes' in the bottom. One will be rather obvious, about the size of a mechanical pencil lead, or just under 1mm. That hole might be in a larger depression of about 3mm, so the hole itself might be disguised a bit. If you look toward the outlet end of the master cylinder about 10mm or so, there should be another similar depression and hole, but the hole will be smaller. On some reservoirs, that hole will be covered by a plastic deflector, so it might not be visible, but it's still there.

That second hole is open to the fluid in the hoses, all the way down to the caliper(s). As the fluid warms up, it is allowed to expand into the reservoir. The hole is located right next to the diaphragm in the master cylinder that creates all the pressure. As soon as you start moving the lever to apply brakes, that hole is covered and you can build pressure to stop the bike.

In the rear master cylinder, it's a similar arrangement, but easier to see and clean. Remove the master cylinder from the bike, along with the reservoir and the connecting hose. It is easier to remove the reservoir/hose/fitting assembly when the master cylinder is off the bike, but you might need to break the fitting loose before actually unboting the m/c from the frame. Remove the reservoir assembly, you can see the two holes in the bottom of the cavity. The equalizer hole is the upper one. It's always at the end that is closer to the calipers.

.

[steve murdoch]

Iso, a couple pics in this thread.

https://twowheeljunkie.wordpress.com/tag/rear-master-cylinder/

[mattd1205]

Had the same thing happen to my bike after I changed out the brake pads. Both brakes ended up dragging after I replaced both pads. I’m pretty sure nobody ever replaced the fluid in my bike so unsurprisingly the the calipers had a lot of gunk in the channels for the seal and the pistons were pitted with rust. Ended up rebuilding the calipers, the master cylinders (probably didn’t need it) and put venhill custom length stainless steel brake lines on. Was definitely worth the peace of mind. I’d atleast inspect the caliper if the master cylinder bleed hole is indeed clear.

[Isoparmesan]

Ok, after looking at those photos (thank you very much by the way), the holes I cleared out are definitely not the famed brake fluid return hole.

This may be kind of a dumb question, but what's the procedure for removing the reservoir? I had removed the two screws inside it, but it seemed very stuck. I didn't want to wrench on it and break the 40 year old plastic. I assume it's not threaded or anything (doesn't look like it from that guy's blog). Is there a gasket under the reservoir? Is there a lip on the plastic that snaps it into place?