Hi this is my first post on the forum so here it goes.
As the quad warms up it seems like the pressure builds on the rear brake and slowes it down what is the best way to deal with this problem. Thank You to anyone who can shed some light on this for me.

 

is the caliper not releasing after it has been activated? does the piston not retract? do you use the foot brake or the left handlebar brake? check both of the activation pistons to see if they're retracting and then double check the caliper for its retraction and then finally check the reservoir level.

 

I would bleed the brake system.
Sounds like there is air in the lines and as it heats up it expands and forces the brakes to activate.

Your pads may also be worn out and after some time the piston is pushed out too far and gets cocked a little preventing it from going back in.