One thing that has not been mentioned is the return springs. If these are new disregard. However, if they are old they could be fatigued and weaken as they heat up. If the brake hardware hasn't been changed I would replace all of the springs. Otherwise I would look at the brake lines picking up heat. I would look at the main line that crosses from the drivers side to passenger side. On my 49 the line follows the cross member and is "housed" or shielded by said cross member. However, is someone replumbed the line and ran it farther forward near the radiator, that could be a problem.
