Front disc brakes locking up, 72 Duster

CHECK THAT THE MASTER IS FULLY RETURNING. If the pushrod is too long or the master is somehow otherwise being pushed towards the front some, the return ports in the floor of the reservoirs will NOT return the fluid, and the system will hold pressure.

YOU ALREADY KNOW it's a pressure problem as you said you cracked the tubes and the brakes released. There is only so much in there that can do so. The master not returning as I mentioned, some problem with the prop valve (new does not mean "good") or bad hoses as mentioned.