1977 GS750 rear brake dragging

[ScroopieNoopers]

Hey Guys, I have a 1977 GS750 that I have been rebuilding for the past few months. So far the journey has been very successful, thanks in large part to the wonderfully helpful and knowledgeable community on this site. So far I have been able to get past any issues I have had by searching and reading the issues and solutions other people have had. But now, after having completely rebuilt just about damn near everything on the bike I have encountered an issue with the rear brake that is preventing me from getting the bike on the road.

I'll start with the issue. While test riding the bike around the neighborhood the rear brake will begin to tighten up and quickly drag the bike to a stop. So obviously the piston is not retreating back into the caliper as it should be. My first thought was that the piston is getting stuck, however, if I relieve the pressure through the bleeder valve screw the brake pads will loosen its grip on the disc and the bike will roll freely. My second thought was the the brake master cylinder is building up pressure and not releasing the pressure on the piston. However, what is perplexing is that I cannot recreate this issue in the garage. If i pump the rear brake in the garage the brake seems to operate normally.

Both the rear caliper and brake master cylinder have been replaced and rebuilt.

Does anyone have any insight into what might be causing this issue?

Here is a picture of the rear brake set up
gs750 rear brake set up.jpg

[mnferwerda]

Rear brake sticking

From your test in the garage it seems that the master cylinder is ok. That means something else is happening while you are riding about.

Hmm, maybe your rear wheel is slipping in some fashion or your disk is moving...

Hey Guys, I have a 1977 GS750 that I have been rebuilding for the past few months. So far the journey has been very successful, thanks in large part to the wonderfully helpful and knowledgeable community on this site. So far I have been able to get past any issues I have had by searching and reading the issues and solutions other people have had. But now, after having completely rebuilt just about damn near everything on the bike I have encountered an issue with the rear brake that is preventing me from getting the bike on the road.

I'll start with the issue. While test riding the bike around the neighborhood the rear brake will begin to tighten up and quickly drag the bike to a stop. So obviously the piston is not retreating back into the caliper as it should be. My first thought was that the piston is getting stuck, however, if I relieve the pressure through the bleeder valve screw the brake pads will loosen its grip on the disc and the bike will roll freely. My second thought was the the brake master cylinder is building up pressure and not releasing the pressure on the piston. However, what is perplexing is that I cannot recreate this issue in the garage. If i pump the rear brake in the garage the brake seems to operate normally.

Both the rear caliper and brake master cylinder have been replaced and rebuilt.

Does anyone have any insight into what might be causing this issue?

Here is a picture of the rear brake set up
[ATTACH=CONFIG]46544[/ATTACH]

[Nessism]

Sounds like the return port in the master cylinder is plugged up. Hope you replaced the brake line when you redid the master and caliper otherwise contamination inside the old line could be releasing crud into your fresh and clean system.

[Big T]

The rubber lines have also been known to come apart internally and create a one way valve

[BurntKittyForge]

... However, what is perplexing is that I cannot recreate this issue in the garage. If i pump the rear brake in the garage the brake seems to operate normally.

Both the rear caliper and brake master cylinder have been replaced and rebuilt.

Does anyone have any insight into what might be causing this issue?...

Maybe heat while applying on road(?)
The makeup of brake fluid is that it has a high boiling point, the main reason for changing it ,it seems is absorbsion/contamination of water...

I had exact same problem except with a front brake on one side a few weeks ago. People I asked suggested cleaning the piston sides in caliper and I did but they were shiny already. could be dirt in between the piston or inside the actual caliper and /or master cylinder(?).

I used a big C-clamp to work the pistons in and out repeatedly, tried pressing both at same time but not too hard, connected a harbor-freight $20 brake vacuum , some odd reason the bleed screw would relieve pressure while stuck but wouldn't bleed otherwise.

It's possible the bleeder bolt threads were so worn that air was getting sucked in(?) I carefully wrapped the bleeder in teflon pipe thread stuff. bled the other brake plenty but still couldn't get it to bleed fluid properly.

Did a "reverse bleed" using an old baby oral syringe dropper, NEEDED/USED safety glasses since first dot 4 to eyeball during ordeal...

Finally with the vacuum connected and pumped to 20(units on gauge) I was about to give up and try a master cylinder rebuild/ complete disassemble anyway, so I squeezed very hard on the brake lever and finally brake fluid drained (shot out into the vacuum tube which I added a clear tube to see what the heck...) I squeezed repeatedly until the filthy looking fluid looked clear (went through couple bottle in whole process bleeding both sides).

The brake finally no longer sticks on that side. My bike has dual discs/ calipers so I am not even sure it's working properly but it hasn't stuck anymore even using heavy brake pressure. I'll remove the other side caliper to check but back then I had had enough messing with it.

On this caliper there is a stainless looking clip that goes under the pad and I noticed IF it moves up a few mm's it would clip below one piston preventing the other from extending and that would lock the brake but don't think that was the problem...

I can tell you what NOT to do is try to ride to pull over unless in immediate danger: First ride I didn't have current MC license OR inspection sticker on bike and road was common place to spot cops traveling,
so when my front brake stuck at light I decided to power through it , through intersection onto side residential road and by then there was smoke coming off the disc, caliper etc.
I'm lucky the disc is ok and not warped but it may have damaged the fork seals, the caliper worse, my clutch, strained a gear(s)(?) and other crap I didn't think about...Oh, the dust seal o-ring had heated and expanded so bad it fell out when I looked ...

Good Luck, God Bless and
Hope you post an update and I'll try and get more info as well.
I'm trying to decide between newer looking caliper from parted bike OR rebuild for that side myself.
Tired of dealing with it so riding with atleast one good disc...

[Big T]

BKF

If you never pulled the piston out and inspected the inside of the caliper, you are doing your self a dis service

You may be surprised what you find in there

[ScroopieNoopers]

I think I may have localized the issue for my particular problem. If I remove the rubber diaphragm from the brake fluid reservoir the issue goes away. This leads me to believe that the pressure in the system is not letting off properly. I rode around with the diaphragm out and it seemed to do ok, the only issue is that the reservoir leaked brake fluid. Now, I didnt have a lot of time to test this fix, as i had to rush off to work. I will fiddle with it some more and see if this is indeed the issue. Has anyone else has problems with reservoir diaphragm?
brake.jpg

Also, the entire system has been blown out and cleaned. Caliper completely disassembled and and cleaned. Brake line replaced. MC rebuilt.

[2BRacing]

I think I may have localized the issue for my particular problem. If I remove the rubber diaphragm from the brake fluid reservoir the issue goes away.
[ATTACH=CONFIG]46564[/ATTACH]

Usually this type of problem is caused by the tiny return port hole in the master cylinder being blocked. When you ride the bike with the brake dragging, it heats the fluid up, resulting in increased pressure and possible brake lock-up. That is why you cannot replicate the problem just pressing the brake in your garage, because no heat is generated.

However, in your case you state that the issue goes away when you remove the rubber diaphragm, so I think it is unlikely that the tiny return port hole in the master cylinder is blocked. Looking at your photo, I think you have the rubber diaphragm fitted upside-down. The flat part that appears to be against the cap should be down in the reservoir, with just a narrow lip around the top of the reservoir contacting the cap.

Also, I am not sure if the rear reservoir/diaphragm/cap on all the different engine size/year models is the same design, but looking at a 1981 650 on which I am working, there is also a thin white plastic disc which goes between the rubber diaphragm and the cap.

Perhaps you can look at an exploded parts view for your specific bike model to see if yours needs the thin plastic disc as well, and how the parts should be fitted.

[GRAND ROUGE]

The reservoir cap / boot assembly has nothing to do with the problem OP has described.

Nessism is on the right track....the return port is either blocked by debris...... or.... the piston assembly is not retracting completely thus allowing the secondary cup to block the return port. Thinking that because M cylinder was removed and rebuilt, and IF for sure the return port was clear, does the master cylinder piston / cup assembly bottom out properly against the washer / internal snap ring when brake pedal released? Is there an adjustment to be made on the push rod itself? If the piston was part of the rebuild kit and if it was replaced, was it the same in every dimension as the original?

[TxGSrider]

If it is a blockage or the if the plunger isn't retracting all of the way, it should be pretty easy to test. Just try to reverse bleed the system, if the port is blocked, it should be very hard to reverse bleed it (or it may even dislodge a small piece of debris if that's the problem).

*** Sorry, just read the post that you already tried this ***

[TxGSrider]

One more thing to think about... if you use a master cylinder from a drum brake bike, it should have a residual valve built in that keeps a small about of pressure on drum brakes. If you use that style of master cylinder on a disc brake, it will cause the disc to drag slightly and as it gets hot the fluid wont return into the reservoir and it will lock up the wheel.

[Guest]

One more thing to think about... if you use a master cylinder from a drum brake bike, it should have a residual valve built in that keeps a small about of pressure on drum brakes. If you use that style of master cylinder on a disc brake, it will cause the disc to drag slightly and as it gets hot the fluid wont return into the reservoir and it will lock up the wheel.

never seen a hydraulic drum brake on a motorcycle.

I'd bet the farm on a plugged compensating port, try a search

[TxGSrider]

never seen a hydraulic drum brake on a motorcycle.

I'd bet the farm on a plugged compensating port, try a search

I honestly would like to apologize for my ignorance on many motorcycle issues. I have spent many years working on old cars and ATVs, but not that much motorcycle work except on the few that Ive actually owned over the years. On ATVs and old cars, there are allot of hydraulic drum brakes in use, and Ive seen folks convert to discs on cars and try to use the old drum brake master cylinders without pulling the residual valve out from the master cylinder and they will behave just as the OP brake. They get hot and lock up. Please forgive me on my learning curve.

[ScroopieNoopers]

Ok. I figured it out. Stupid me did not adjust the length of the master cylinder plunger rod (or whatever it is called). It was too long and not allowing the proper range of travel. I have since adjusted to the proper length and everything works great.
gs750 rear brake set up.jpg

[Steve]

Sounds like the return port in the master cylinder is plugged up.

Usually this type of problem is caused by the tiny return port hole in the master cylinder being blocked.

This is the usual cause of a brake locking up.

Ok. I figured it out. Stupid me did not adjust the length of the master cylinder plunger rod (or whatever it is called). It was too long and not allowing the proper range of travel. I have since adjusted to the proper length and everything works great.

This will also affect brake operation, but it seldom happens, because the length of the rod is almost never changed.

Bottom line: the root problem is the same. The return port was blocked.
The reason it was blocked was different, but either way, warm fluid was not allowed to expand into the reservoir.

.