Rear wheel locked up on me

I have an 08 GRiSO 1100 which had sat for a few years, only 10k on it. I have worked out all the bugs and got it on the road. After about 20 miles of riding, the bike started to bog down. I got off the throttle and came to a stop. The rear wheel had locked up. I put it in neutral and couldn't even push it off the road way. After about 5 minutes, it rolled perfectly. I think I had a small issue like that before but blew it off. I rode home, never touching the rear brake and had no issue.
So the question is, with it working fine, how do I know if the master cylinder or caliper. I am ok replacing either, just would rather get it right the first time.

Thanks
Rick

What it will be is the adjustment on the plunger that operates the rear master cylinder. This has a thread at the foot lever end and it is wound too far out so the piston of the master cylinder isn't able to return to the end of its travel. Because of this it, the piston, isn't uncovering the priming port that allows fluid to enter or leave the brake circuit back to the fluid reservoir. This means that the brake system is closed and as the fluid heats up and expands by application of the brake it can't depressurise the system by releasing fluid to the reservoir so it expands the only way it can, bu pushing the pistons in the caliper out and applying the brake. This creates more heat, causing more expansion and applying the brake harder until, as you have found, the wheel will lock.

Unwind the plunger a couple of turns and make sure that there is free play between the end of it and the piston of the master so the piston can return fully and the problem will be cured. Sadly, if this has happened a few times now it may already of destroyed your rear disc. Check it carefully for blueing and warpage.

Also brake hoses can deteriorate from the inside out and give similar symptoms.

Pete, could it be that since the bike has sat for a long time, the fluid needs changed/flushed? The return port might simply be clogged because of the old fluid?

Could be, but I doubt it. If the port is blocked by some sort of foreign body it would have to be *Inside* the master cylinder and not be moved off the port when the piston and seal passed over it on application of the brake or always return to exactly the same spot when the lever/piston is released.

Not really tremendously likely I wouldn't of thought.

Thanks Pete. I adjusted the rod out so I have a bout 7mm of play before it comes in contact with the master cylinder plunger. I decided to put new pads on at the same time. I have a question as I do ntoo know the inner workings of the mater cylinder. Should I be able to push in the pistons while everything is connected? They would not go. Once i disconnected the fluid line, they pushed in with no resistance. Anyways, put the new pads in, bled the line and all works. Just doesn't seem as though I have a lot of pedal.

7mm is way, way too much. All you want is barely detectable clearance between the plunger and the piston.

Having said that if the pistons wouldn't go back into the caliper until you broke into the circuit it speaks of a deeper problem, the one suggested before of a blocked priming port. This only tends to happen to machines that never have their brake fluid changed and thr Uber of the piston seal degrades and begins to break up. Then a tiny piece may get lodged in the priming port preventing fluid return to the reservoir. As I mentioned before I found this to be an unlikely cause but given your new experience it sounds like it may fit the bill.

I don't know if master cylinder rebuild kits are readily available for the master? Might have to buy a new one. Does the caliper still float on its pins?

Thanks, I will close the gap. I thought I read somewhere between 5-10 on the play. must have gotten that wrong. The caliper has full movement on the pins. Old pads were worn very evenly. For the money and time involved, I may just get a new master. Before bleeding by the pumping method, I tried my Mighty Vac, I couldn't pull anything thru.

I mentioned the possibility of a blocked port because the bike had sat for so long, so the fluid was a probable culprit. I fiddle about with old bikes a bit, and this is a common issue when they sit.

You might try using a single wire pulled from a wire brush to dislodge whatever is plugging the return port. Flush and give it another bleed cycle.

Pretty difficult given the right angle fitting for the reservoir hose.

New master cylinder installed, all problems resolved.
Thanks all.