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When you can't build up air pressure AND the air seems to be escaping from the black nozzle at the bottom of your air dryer, that usually means the purge valve is bad or being held open by debris (from a broken turbo valve or trash).  The repair is to overhaul or rebuild the air dryer.  But getting stuck with 40 or 50 psi of air and unable to build anymore to get the rear spring brakes released creates the need for a big tow truck or low boy flat bed tow truck . . . unless you happen to have a female 1/2 inch pipe coupler.  That will look like a piece of pipe that has threads inside of each end.  The threads are pipe thread.  First, remove both inlet and outlet hoses from the air dryer and both 1/2 inch brass fittings that screw into the air dryer.  Then use the coupler to connect those two hoses.  This will allow you to make and hold air.