Another rear brake sticking question

Today my rear brake stuck on. It's not the first time it's happened, but today was the worst. The pedal went really solid and the caliper was smoking.
Sometimes there seems to be more play in the pedal than others. A few weeks ago I adjusted it to take some of the play out and it was good. But then tonight on my way home from work it went solid, and I couldn't prise the pads back. When I got home and it had cooled down I could prise them back no problem, but there was more play in the pedal than before (I guess the fluid had boiled) when I pumped it back up.
The bizarre thing is that it only seems to occur when the weather is hot. I've had no bother with it all winter, but it did play up a little last summer.
So to cut a long and boring story short my question is this: is it the master cylinder or the caliper?

Clean it all, and change the fluid - sounds like a build up of crud.

http://www.advrider.com/forums/showthread.php?t=59077&highlight=rear+disk+temperature

Read and do! If it's relevant? as you ain't said what the bike is?
It's easier than it looks.

boiling point

The adjustment you made probably had something to do with it.
Free play is needed [ a little ] at master cylinder ,this stops any pressure down the brake lines,continual pressure will cook the caliper and create enough heat to boil the brake fluid producing a steam that will turn eventually
to somthing close to water. I not sure But maybe the steam or gas itself created an expansion in system forcing the caliper to operate.

GS 850

You could always put a bid on the Suzuki GS850 on Ebay, its a 1979 with 1 (one ) mile on the clock.
Imagine, a reliable GS for £4000 !

check that the metal plate that the brake pad material is bonded to isn't in contact with the brake caliper itself at the edges. I replaced pads and the back edge just touched the alloy frame of the caliper causing it to bind on with red hot discs etc the result. Filed off a millimeter or so from the back edge of the caliper and all moves smooth again.

Thanks for the replies. I do all the cleaning and lubing malarkey usually (not too often mind) but perhaps I did take out too much free play. It's unlikely though because I've been riding around for a few weeks and it's been fine.

I'll look out for the pads catching the calliper/pins. That's a new one on me.

It's a 650 Dakar BTW, but the principles in the ADVRider thread still apply even if the details are different.