1979 GS550E Front Brake Lock

[Guest]

Hey I'm new to the forum and motorcycles for that matter. I bought a 79 GS550E and it works great. The other day however the front brakes locked up on me. Figuring it was the return valve for the brake fluid I bought a master cylinder rebuild kit in order to fix it. The bike worked great for the next 150 miles. I was out today though and I felt the front brake tightening up again so I pulled over and sure enough the front brakes were starting to lock up again.

I'm trying to find what the cause might be. I opened the bleeder valve today when the brakes began locking and the pressure let up so I'm assuming it has to be the fluid not being able to return. I read that the brake lines sometimes can get clogged but I shot air through them when I was rebuilding the master cylinder to check and there was a decent flow. I don't know where else brake fluid could get jammed. Maybe I'm missing the problem all together though I'm not much of a mechanic.

Thanks in advance for the help!

[mmattockx]

There is a small hole in the master cylinder bore that lets fluid return to the reservoir when the lever is released. My money says that return hole is blocked.


Mark

[confusedcyclist]

That's what I was thinking too but I replaced that part when I rebuilt the master cylinder. Could it have gotten blocked already?

[fastbysuzuki]

Did you rebuild the calipers? They could be crud in there and it has come lose and blocked the release hole in the master cylinder.
But if it is blocked then I would expect the brake to more or less lock on the first time you apply it so to check get the front wheel off the ground apply the brake a few times and check if the wheel rotates freely if not the hole must be blocked or the pistons are too tight in the bores and require stripping.
if the Pistons are seizing it could be a heat problem, generating enough heat to expand the brake fluid to the point it has nowhere to go other than pushing the Pistons out thus applying brake pressure.

[2BRacing]

There is a small hole in the master cylinder bore that lets fluid return to the reservoir when the lever is released. My money says that return hole is blocked.


Mark

That's what I was thinking too but I replaced that part when I rebuilt the master cylinder. Could it have gotten blocked already?

You cannot "replace" the return hole in the master cylinder with anything in the kit!

You replaced the piston, spring and rubber parts only. If you look at the bottom of the brake fluid reservoir you will see a relatively large hole that feeds the brake fluid out to the system. There is also another TINY hole (about the size of a pin) near that larger hole, but more towards the brake line end of the M/C, which allows fluid to return to the M/C.

That hole is clogged and must be cleaned. On some M/C's, the fluid reservoir must be removed in order to see the small return hole (note that there will be an O-ring between the fluid reservoir and M/C body, which should also be replaced to prevent leaks.

[hillsy]

If your brake lines are the original ones, they are probably breaking up inside and contaminating your fluid with bits of rubber / gunk.

You should probably replace these with braided lines (and if you don't know how old they are, then replacing them is just a good idea anyway).

[Nessism]

Please check the Newbie Mistakes thread linked in my signature. How to tend to the brakes is covered therein.

Basically, you need to perform a full brake system tear down and clean out. Replace the caliper pistons and brake line, in addition to your recently replaced master cylinder parts. Also, the master return port sounds clogged still so you might want to reclean the master while you are at it. Oh, and do the back too since it's likely full of sludge as well.

[confusedcyclist]

I did clean out the calipers the first time around and everything looked to be in good shape nothing was corroded. I think I'm going to try and replace the brake lines.

I am still confused on the return port though. I did not see a return port in the mc, maybe I'm just missing it. I can't get the reservoir off and I'm afraid to use more force on the chance of breaking it. I looked in from the bore and I could not see a hole either. There are four small holes on the piston and I assumed these were the return ports but we all know what we do when we assume... Am I just missing the return port?

[tkent02]

Yes. ..

[smilinbrad]

I had front brake issues on both of my Silverwings. I bought the kit to clean and rebuild one. it was a mess and I got brake fluid on my fairing after parking in the sun. NOT recommended.
I bought an inexpensive NEW MC on-line. much better idea and I know all the openings are clear. only cost an extra $20 or less over the re-build kit.

[confusedcyclist]

Ok I finally found my return port... Problem is cleaning it... The reservoir is being stubborn and won't come out 😒
Anyway thanks for the help!