| On the ST (and maybe all Dukes) there's a screw insert on the front brake and clutch levers. It looks for all the world as if it's purpose is to adjust the span, but I know that if you dick around with it, the master cylinder piston doesn't come back far enough to let the fluid return when you let the brake off. That's in laymans terms, it may not be what really happens but the result is that the brakes tend to 'pump up' until they lock on.
I reckon that the rear master cylinder must work the same way? If any adjustment is stopping it return properly, the rear brake is going to drag pretty badly. If you think about it, brakes are more powerful than the engine, because you can slow down quicker than you can accelerate, and they do it all by using friction to turn the energy of a rolling bike into heat. That's a lot of heat. It wouldn't take much of a dragging brake at a constant 80mph to make the brakes red hot, wear down the pads and start weld metal to metal | 
Jools
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