My track bike is an 848.
I recently boiled the fluid in the rear caliper and destroyed the caliper.
The pads were completly gone, and I never use the rear brake, so I figure they were dragging. I adjusted the pedal so there was 1cm of freeplay at the pedal.
I replaced the caliper, fluid, bleed the system and everything seemed great.
Last weekend I came off the track and while putting the warmers on i noticed the caliper was very hot and the brake was dragging. All that 1cm freeplay was gone.
I adjusted the pedal again and seems to be ok. But 40 min later I have almost no rear brake...
Anyone else have this problem?
Any possible solutions?

This might sound a bit off but is there any chance your boot is touching the pedal. Sometimes when folks lean off they have their feet in places they were not intending. I had a pair of Alpenstar boots once and found I was hitting the brake lever with one of the many plastic protrusions on the boot.

One other thing to check is the possibilty of a heat shield for the brake master cylinder attached to the pedal. The heat from the exhaust might be heating up the fluid. Check the master to see if there may be a small piece of dirt blocking the orifice for the return of the oil to the resevoir.
Cheers

On a street bike I had to take the rear apart before and clean it to stop the caliper from sticking. It was just collect to much fine dirt and stopped the piston from retracting the requisite amount. Popping the pistons out and clean everything solved the problem for me.

Make sure that your reservoir is not at the MAX line, rather it should be at the MIN line or sligthly above. If the reservoir is full and things heat up, the fluid expands and you get hydraulic lock and that's not good. Ultimately your brake drags and voils, the symptoms that you describe.

I hope that this helps.

The return port in master will not be open unless piston fully settles in released position. Free play needs to be measured between rod going into master, and piston. Only 2-5mm needed there. there should be a mechanical slop before the play or movement of the piston.

The dimpled ring that goes into the reservoir bladder before the cap allows leakage before pressure can build. If that was left out, yes too full could be a problem. If you look at any master cap, there are cutouts or dimples to not create a perfect seal. Enough seal to hold fluid if the bike is on it's side. Not enough seal to hold pressure.

I think your setup is blocking the return port, or the master is sticking and not returning to internal stop.