Q: rear gas tank built up pressure leaking gas out gas cap

There was a recall back in the early 90’s for this problem. The cause was the fuel pumps would use the fuel and then return it to another tank through the switching valve and the fuel pumps. The repair was to replace the switching valve on the frame rail and the add a check valve on each fuel tank pressure line. If you replaced the fuel tank in the middle and did not reuse the check valves or they were never installed then the dealer should check to see if the recall pertains to the vehicle and is still active.