Just got to Manc to find upon Stopping I had no air, was fine on the way, stopped in the warehouse and lost half immediately, tried to build it up to move but nothing,
Compressor■■?
Knock your handbrake off and see if it builds up then. If it does you’ve got a knackered valve or chamber.
If that works pull your airlines off and try it again with the handbrake on to see if it’s trailer or unit 
Could be compressor failed though if losing it that quick most likely will be exhausting it straight out the drier bar some other major leak in the system.
If it’s the drier you’ll usually hear a fairly low pitched hiss when the engine’s running as the exhaust port is usually quite a large bore and is often not detected as masked by engine noise.
Try applying service and parking brakes with engine off and listen for air loss from usual suspect brake components and drop susies to isolate trailer and establish if it’s unit or trailer. I would be thinking compressor or drier if you still can’t build enough to move by revving the ■■■■ of it.
Sounds like you have an air leak.
Somewhere.
Pulled the lines, had enough air to pull the wagon out the way using the shunt button. Fitters been, replaced the air dryer filter as the old one was full of water, builds up again but stupidly slow, enough to get me back up the motorway but will need looked at when I’m back, he says more than likely the 4way valve and everything beyond the filter is also full of water.