Did you read his post? It is a 1990! Does it have an air compressor? If it does and it is engine driven, and water cooled that could be the source of your leak. You may only see a pressure drop while engine is running. Try putting the pressure tester on the engine at normal operating temperature and no pressure on the gauge. Start the truck and boost up the idle to about 1000 rpm and let it run. If the pressure builds to above normal pressure, there is an internal leak somewhere. Some leaks will not surface unless the engine is running. Also check the injectors and cups by draining the oil (water mixture), pressure test the system to see where the leak is coming from.