i have a f-150 2002 the rear breaks heat up about every year I have replaced the master cylinder every year that seems to help for a year then it starts to heat up and smell again is there another reason for this to happen

 

Your rear calipers could be partially seized, the brake hoses on these trucks rot from the inside out as well so your brakes may not be fully releasing. The emergency brake system is garbage. The cables seize very early on and there is a scissor like actuator that forces the e-brake pads out that rusts solid not allowing the e-brake to fully release. My truck only has about 78,000 miles and I have had 3 out of 4 of these issues already, my rear calipers are still ok.