Rear Brake Intermittant Seizure, and Hello

[thedonuk]

Hi all, i am new here, my name is Don, i live in London, UK.

Recently picked up a '94 GS500E for £250, was not in perfect condition but for the money was fine, the carbs need tuning, and a couple of other minor things(such as speedo light bulb), i have replaced the chain/sprockets, front brake pads and was given alot of spares with the deal (2 spare wheels, 2 spare front forks, swingarm, engine, bodywork etc etc).
but the main concern of mine is an intermittant rear brake seizure i get from time to time.

It first started when i was riding the bike reasonably fast, the rear brake started dragging, it came on progressively untill i had to be well over 8k revs to move off from first.... at this point i thought i should stop, and at least see whats wrong... to cut a long story short, my mechanic has bled the breakes to death (literally, i have sat with him for over 4 hours as he did it... never seen such perseverance, quite inspiring actually...), changed my rear brake hose to a goodridge race one, but i still  get this intermittant seizure, i dont think its the caliper or the pads, or any blockages in the hoses/resevour, however i am suspect of the rear master piston, as when the brakes seize the rear brake pedal is unable to move at all, also if you undo the bleeding nipple oil spurts out as if its under pressure, i have had all sorts of reasons as to why this is happening thrown at me, from the return valve not working, to not enough play on the rear brake lever, i am at the point now of just buying a new rear master piston... unless anyone has any ideas or previous experience with this, thanks in advance for any help.

-TheDon

[ukchickenlover]

I had a problem like this when I did not use my bike for a few weeks. For me the the brake pedal itself was sticking. I sprayed wd40 onto the pivot point and moved the pedal up and down then left it for a bit for the wd40 to act on the rust then repeated this several times.

[Simba]

Check to make sure that the rotor is not warped
also make sure that the slide pins (whats the pads move in and out on) are free to move around
and see if there is some damage to the calpier mount

[scratch]

Welcome to GStwin!

Otherwise, change out the brake fluid; it may have too much water or air in the system, and when it gets hot the air/water will expand causing involuntary actuation of the brake.  What color is the fluid in the resivior (which does not indicate what color is in the rest of the system, but it's a start)?  Brake fluid should be changed every 2 years.

[thedonuk]

First would like to say thanks for all the suggestions,
Throughout all this troubleshooting process, we must have been though 5-6 complete breake fluid changes, i did have a concern that due to the hydroscopic nature of the brake fluid, while the reseviour was open water may have got in, my mechanic assures me its not that... as a last resort i will completely change it again, he also says the caliper is fine. I will try lubing the brake pedal as i understand if it is even a milimeter depressed you gonna have problems. Any other suggestions warmly welcomed...

[scratch]

Cleaning the caliper brake pistons.

[gsmetal]

I'm going to disagree with your mechanic and tell you to rebuild the rear brake caliper (easy fix!).

My bet is that the dust seals need replacing. I had this exact problem with a 87 FZ750 - the seals were shot and the brake fluid boiled to death it wouldn't allow the pistons to retract back into the caliper......I was driving home when it happened as night and I looked down to see my disc was bright orange from the heat with flames coming from the caliper (cool!)

[thedonuk]

Yea he already rebuilt the caluper and it did have that problem, a load of crap trapped between the rubber and the piston... the pistons retract fine now. But my suspiction is still with the rear master piston... will update when/if i sort this...

[gsJack]

If the problem still persists, carry a 8mm box wrench with you and next time the rear wheel starts to lock up throw the bike up on the stand and open the bleeder screw.  If the wheel is still locked the problem is most likely in the caliper, but if this frees the wheel the problem is most likely in the caliper.

[Scorpio65]

Its werid, but i had the same problem on my car. It turned out to be the master cylinder. Once it locked up, i had to losen the brake line at the master cylinder to remove the pressure. the faster i went the quicker it would lock up.

[scratch]

So maybe there's some junk blocking the return passages for the master cylinder (like old rubber that's floating around from a disintegrating system).

[thedonuk]

If the problem still persists, carry a 8mm box wrench with you and next time the rear wheel starts to lock up throw the bike up on the stand and open the bleeder screw.  If the wheel is still locked the problem is most likely in the caliper, but if this frees the wheel the problem is most likely in the caliper.

Funny, thats exactly what i have been doing, people keep asking me why i have an adjustable in my pocket
Scorpio, thats what i find also, if im riding hard it happens faster/more often.
Scratch, so an overhaul of the master cylinder you thinking?

[scratch]

Yes.  Or, at least a backflushing (I don't know how; maybe with a MightyVac (or is it MittyVac?)), otherwise, yeah, a rebuild.  Got a question to ask - Was there little black things floating in the fluid?  Did you see any?  Maybe a bunch settled at the bottom of the m/c?  Or, flowed out when bled/flushed?  That's indicative of deteriorating rubber in the system; the rubber could come from anything, anywhere; the o-rings on the m/c piston/plunger, the lines themselves, the rubber o-ring seals around the pistons.

[thedonuk]

I dont think any rubber pieces came through, i know tho on the original fluid change there was alot of crystals that came out...

[Gisser]

Sometimes the pushrod that actuates the master cylinder will become corroded and will stick causing the brake to lock up.  Easy to sand or buff it off if that's the problem.