Don't panic it's not the unload valve bellows that has split this works perfect (well it did before all this cold weather) the valve that has failed is the pressure reducing valve mounted on the end of the tractor service tank. My explorer is an early one with a three tank setup, main,tractor and trailer. The pressure reducing valve drops the air pressure to about 85pis in the trailer and tractor tanks while the main tank runs about 115psi (from memory) I can look for a part number in the book when i have access to it, Cheers.