Thanks Gary. I have no pressure on the drivers side, confirmed. The whole system WAS operating until recently. I haven’t disconnected the line entering the pressure relief valve and tried to put my thumb over it, but when I push on the test port and hear the Pfft, pfft it is actually pushing air out. It is not a port that I can plug or cover with my thumb, so I can’t do a build up check there, but I’ll pull the incoming line tomorrow and do that. How long/ how many strokes should it take, in general, before it builds enough pressure to push a finger off?Just because you are getting the pfft - pfft sound doesn't mean that your compressor is building up air pressure. I would recommend disconnecting the crossover line you replaced. Disconnect it at the drivers side. Run the engine and place your finger of the end of the line. You should feel pressure build up which will blow your finger off the end of the line. If you have pressure there, you likely have additional corroded air lines and/or one of the small regulators near the air tanks. If there is no pressure there disconnect the pressure line coming from the compressor. Disconnect it at the pressure regulator. Run the engine and place your finger over the end of the line coming from the compressor. You should feel high pressure build up which will blow your finger off of the end of the line if the compressor is working. If you don't feel high pressure against your finger, the next step is to remove the output line from the compressor and do the same thing with your finger directly on the output port on the compressor. If you have pressure at the compressor, you likely have a leak in the line from the compressor to the regulator or the line is corroded closed. If there is no pressure build up, your compressor is faulty.
A separate suggestion is to remove all three drain valves and clean out the crud. This won't help your current problem, but should be done if you haven't already done so.
And yes, I have cleaned the tank drains, but I am going to replace two of them as soon as I get the system working.