Front brake seizing up

[Anonymous]

I recently bought a '78 GS550E. The previous owner had it for a little more than a year and 5,000 miles and said he did nothing but ride it - no maintenance at all. So after changing the spark plugs and points and putting in synthetic oil and a new oil filter (which made a night-and-day difference), it's riding well.

Every few days, the front brake seizes up - this happens regardless of if it's just sitting or how much I'm riding it. The caliper grabs the disc tight as though I'm squeezing hard on the lever, and the lever gets extremely stiff. The fix is to loosen the union bolt where the brake line meets the caliper, which always squirts a few drops before the extra pressure is out of it, then tighten it again. I would loosen the brake bleeding nut to do this, but the hex part is nearly rounded off so I use the much larger union bolt. Then it's fine for the next few days. Yesterday I put on new front brake pads and pumped a lot of brake fluid through the system and made sure it was all new fluid and properly bled, then this afternoon just with it sitting there I could tell it was getting pretty tight again.

What the heck is going on? For some reason there is pressure building up in the system, starting at the master cylinder, since there's pressure through the whole line when it's seized up, and loosening any of the union bolts relieves the pressure and makes it work just fine for the next few days. It's not due to something besides brake fluid in there, because it's doing it just as much now after having pumped a lot of fluid through the system and having pure clean new brake fluid in there.

Anyway, your help will be greatly appreciated.

Alex

[Dink]

If you look inside the master cylinder there are two holes in the bottom the one closer to the brake union joint is a return hole it sounds like it is at east partially obstructed, thereby not releasing the pressure. I would get a master cylinder rebuild kit and clean out and rebuild the master cylinder.
Dink

[KEITH KRAUSE]

It may be what Dink said. It may also be trapped air in the master cylinder. After checking your reservoir, try bleeding the master cylinder first, followed by a regular bleed. This method used to be in the "Tips and Tricks" section and may help you. With the union bolt off at the master cylinder, squeeze the lever in and hold it. Plug the union hole with your thumb and release the lever. Squeeze the lever again as you release your thumb. Repeat this until you don't see any more air bubbles come out.
Be sure to have brake fluid handy in case the reservoir gets low. Then complete the bleeding the regular way at the caliper nipples.

[Don Lobacz]

What Dink said is the most likely cause of the problem. The most forward of the 2 holes tapers down to a tiny pressure-release hole. This can often be opened up by jabbing a soft wire into it. New brake fluid tends to break up and wash away old buildup, so you may be OK after opening this hole.

[Anonymous]

Thanks for the help and suggestions, guys. I talked to my friend, an experienced car and bike mechanic, who was pretty sure it was the same thing. He has a set of miniature drill bits, and used his smallest one, which looks like just a piece of wire, to jab and poke around in the return hole in the master cylinder, and that opened it up. After running more new brake fluid through the system, we sanded the disc (it had gotten glazed from periodically seizing up while I was riding) and I took it out for a ride for about an hour. The problem is totally fixed now, not only does the brake let go properly, but it also grabs properly now - with new pads and a sanded disc, I can fully compress the front forks with just medium pressure on the brake lever. Replacing the front fork seals and fork oil is something else this bike needs badly, but it's a job for another day.

Alex