Vauxhall Zafira brakes drag occassionally

There seems to be a problem with my braking system. When driven for a while the brakes start to drag, the handbrake is not the problem but the hydraulic system pressurizes and the foot pedal is hard at the top. There is no fluid leaks. after leaving to cool down, everything reverts to normal. The problem is fairly intermitant and in varying degrees but has resulted in a hard shoulder/smoking rear hub fright recently.