Ok i have a wierd problem i think. While towing a light load on a 25 foot gooseneck trailer i noticed my cooling system was not holding pressure yet i have no coolant loss or noticeable external leaks. The next day i pressure tested the system with a buddies fancy pressure tester and also tested the radiator cap and it was holding pressure,but still when i drive there is no pressure on the system. Has anyone ever heard of this?