you need to strip down the calipers and make sure they are completely clean of rust or and brake dust sitting where the caliper should be sliding when in operation.the caliper needs to be able to move in and out on the sliders so make sure that they have grease inside the rubbers housing the sliders.
if it is all clean then you need to make sure the hanbrake mechanism is not sticking and use some copper grease to lubricate it
hope this helps!