The outflow valves on a KC-135 aircraft, which are used to regulate the pressure of the aircraft, were capped off during a 5-year overhaul and, for some reason, were not reopened. A civilian depot technician subsequently performed a pressure check on the aircraft with a homemade gauge and without a procedure. The gauge didn't have a maximum peg for the needle so the technician did not see the needle go around the gauge the first time. As the technician continued to pressurize the aircraft with the needle on its second trip around the gauge, the aircraft exploded, blowing the rear hatch over 70 yards!