front discs locking up

hey everyone. I got a 75 dart sport with manual disc breaks. after about 10-15 min of driving the front breaks start grabbing to the point the front end starts to shake and I have to pull over to let them cool down. Wait 10 mins and off I go. calipers pads and rotors were all replaced new a couple years ago but the car doesn't get driven much. I took the calipers off and the piston is moving fine. Could it be a flex hose failure???Any help would be greatly appreciated.

thanks