This will mean that the tank will fill from the truck compressor and then when up to max pressure will disconnect the valve, meaning unless you use it you will always have 60lts of air at 120psi.
I can think of a few reasons why this would be useful, if for example the truck was inadvertently left in gear for a few days and the air drained out (as it does) you cant get it out of gear because both the clutch and gear change are air operated, and of course you cant start it in gear with the brakes on.
So at this point all you have to do is flick a switch, air from the tank fills the truck tank and everything works.
I have also positioned two euro air outlets near the back wheels so i can use the air for tires,
In addition i am going to fit a small air compressor in the body that is also connected to the 60ltr tank, this will give me tipple redundancy regarding air. In the event of the truck compressor failing, the electric one would run the brakes and everything else.