In my opinion, for what it's worth, the brakes shouldn't cause the engine to over heat. That kind of drag shouldn't be any worse than pulling a trailer or hauling a load. I would look for a plugged radiator and or a head/head gasket problem. A proper diagnoses should be done instead of just hanging parts and hoping for the best.
If the brakes have just recently been done, my money is on being adjusted too tight. When I do brakes, I go for a drive afterwards with a brake adjusting tool on board. If I feel them starting to drag and smell like a burnt clutch, I pull over and back them off until they don't drag.
Without being there to see it first hand, this is just an educated guess on my part,
Good luck,
Scott
