Rear brake issue

[Drunken Monkey]

So technically it's not a Monster issue (KTM Duke) but brakes are brakes.

Recently my rear brake has started locking up. The symptoms are that the rear brake begins to drag, heats up and then locks solid within just a few minutes of riding.

Doesn't happen at slow speed, but only when cruising above about 40 MPH for a sustained period of time.

Pressure sensitive brake switch activates, leaving the rear brake light on, and it's easily "cured" by cracking open the bleeder valve.

Piston moves normally, provided the brake bleeder nipple is loose. And the brake operates normally most of the time.

I did this when I first got the bike, and I've since done a complete flush of the fluid and put in new pads. But after a week or so, the problem is back.

I'm 99% certain the problem is the master cylinder not letting fluid back into the reservoir (since the piston will only move when the line is "open"), but I figured I'd check here before the hassle of doing a rebuild on the master.

Any other possibilities?

[Speeddog]

Usual cause of this behavior is no freeplay between the pushrod and the piston in the master, such that the piston is covering the bleed port.

Or the bleed port is plugged.

[Howie]

Another cheaper possibility is the reservoir is overfull, leaving no room for expansion.  Check both, if in doubt add a little free play,  You might also want to check and make sure the caliper piston(s) and pads are not binding.

[Drunken Monkey]

Usual cause of this behavior is no freeplay between the pushrod and the piston in the master, such that the piston is covering the bleed port.

Or the bleed port is plugged.

Good suggestions. But the first thing I did was check the reservoir and make sure there was free play in the pushrod.

Unless you mean something other than "brake has no free play before engaging." in which case please explain?

And by "bleed port" do you mean the part that lets fluid flow back into the reservoir? I have no idea what it's called, but that's what I think the problem is 

[Howie]

That be it (bleed port).  It could clog, but that is rare, particularly since most of us change our brake fluid yearly, if not more often.  More common is the seal in the master is blocking it.  If you have a clear reservoir you should be able to see the fluid return after releasing the brake.  If the rear brake overheated bad enough caliper seals could be damaged.

[Drunken Monkey]

Bought the bike (an '02 Duke II) with a mere 1000 miles on it. So I'm thinking "pockets of brake fluid with the consistency of root beer" isn't impossible as I doubt the fluid had been changed. Then again, the bike was otherwise in great shape... who knows.

I'll give it yet another flushing, and look for fluid returning to the rsvr before going the full rebuild route.

Thanks