Rear caliper sticks after new SS lines and rebuilding MC and caliper.

[Guest]

Yesterday I decided to tackle my rear brake issue. The issue is the caliper sticks and will not retract after a few miles. Once I open the bleeder valve, it will release. Did you know that DOT 4 burns green on a hot disk??? Here is what I did yesterday, which did not resolve my issue.

Took off MC, lines and caliper.
MC- took everything apart, ran through a solvent parts washer, then rinsed in water and dried with a air compressor. used rebuild kit and reassembled

Lines- replaced rubber and hard line with Spiegler SSB's

Caliper- removed, dissembled, cleaned in solvent tank then water, dried, installed rebuild kit. pucks look great, no pitting.

after everything was reassembled and bled, I had the same exact issue. The rear brake is not releasing. I am thinking my MC will need to be replaced. Somehow the fluid is not able to return allowing the pistons to contract.

Does this sound like the correct next move? I would really appreciate some insight on replacement rear MC's, preferable new if at all possible.

[chuck hahn]

2 things...did you poke the little return port hole in the master to be sure its open???Did you scrape all the crud from the groove the caliper piston seals goes in.....did you even remove the seal??? These are the two leading things that make brakes lock up.....clogged return port and scale built up behind and around the seal in the groove.


EDIT;;; be sure the cups are facing the right way as well in the master.

[Strobot]

I poked and blew air through the port hole. I removed the seal and put on a new one from the rebuild kit. I did not scrape the crud off though. A good bit came off but it was not bare metal. I will give that a shot. If crud were the issue, would opening the bleeder still relieve the pressure?

[chuck hahn]

Yes...then piston will move a few thousands and let the rotor roll. I use a Dremel and one of their little nylon wheel brushes to polish the scale away. Doesnt take pressure though..just some carb spary and polish the crud down to base metal. I also run the seal through my thumbnail to flake off the crap that will be stuck to it. If the seal is pitted ANYWHERE replace it so there are no air leaks.


New crush washers too..the old ones typically wont reseal well..and again an air leak source. Once the seal is seated well back in the groove youll really feel the huge difference when you reseat the piston.

[Strobot]

I assume you are talking about the two ring seals in the caliper. I will definitely take another look and clean away. I did have to use a c clamp the reseat the pistons in the caliper. Should it slide in only using hand pressure?

[chuck hahn]

The rubber seal between the piston and the carb body..1 seal on each half of the caliper body as there are 2 pistons. I havent had any that wouldnt go in with just a stiff push with my thumb. Not loosey goosey but not that youd need a C clamp!!!! Should be resistance but you should be able to push the piston in relatively easy. Hard to find a way in words to explain it but I think you get the idea.

[chuck hahn]

Wipe the seals with brake fluid when reinstalling like the manual directs and also a wipe on the piston.

If you dont have one, heres the manual..

http://www.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf

[Strobot]

Got it! Thank you!

[chuck hahn]

Better clean the dog snot out of the rotor and pads with brake cleaner spray. By that I mean like half a can per pad to be well well sure any fluid contamination has been eliminated as good as is possible. Then dry them super well with compressed air. You DO NOT want contaminated pads or discs.

[dorkburger]

All good advice from Chuck. I had a problem in the M/C on mine. The bore was slightly corroded and the circled part was rusted preventing proper movement. After cleaning it all proper operation was restored. My problem occurred after changing the fliud (first time ever for the rear). I figured the binding was caused by the parts moving beyond where they had been for 30 years.

BTW, hows the riding around Frederick? I rode down there for work last year for a week long training course. It seemed there was potentially some nice riding, but I kept running into developments and traffic after finding what seemed to be a decent road......

[Strobot]

The riding in Frederick is great. There are lots of back country roads, that twist around farms and over the hills. I am planning a through Shenandoah National Park then continue on the Blue Ridge parkway for a few days next month.

[Nessism]

If you used aftermarket brake system seals that could be contributing to the issue too. I've had issues with K&L brand brake system parts on more than one occasion in particular.

[Strobot]

I could not locate an OEM rebuild kit. Any possible info on locating one?

[chuck hahn]

http://www.partsoutlaw.com/oemparts/#/l/suz/50d3fef2f8700230d8b4dd38/1977-gs750b-parts

Go here..select REAR CARIPER ( gotta love the Japanese English..LOL ). Item 1-2 is the full rebuild kit and as youll see, the seals are available as well. Doesnt matter what 77 750 you order from...all the same parts.

[derwood]

I poked and blew air through the port hole. I removed the seal and put on a new one from the rebuild kit. I did not scrape the crud off though. A good bit came off but it was not bare metal. I will give that a shot. If crud were the issue, would opening the bleeder still relieve the pressure?

Not sure if you realize that there are 2 ports in the Master Cylinder, a fill port and a compensating (return) port. The compensating port is tiny, smaller than a wire brush whisker, it may be plugged.

Before you tear it all apart again, just take it for a ride and don't use the back brake.
See if the back brake starts to apply.Stop and check to see if the rotor is getting hot. Crack the bleeder to see if it starting to pressure up.

what happens is.. as the fluid heats up from normal brake drag, it expands and starts to apply the brake.
a) the compensating port is plugged
or
b) the master cylinder piston is not positioned correctly, (because of linkage / push rod issues). the piston is blocking the compensating port.