Thanks for all the good suggestions.
I replaced all the check and safety valves on the compressor. I also
added a second unloading port to the other head
where the recycled air line (the line with the cooling fins) is. I
teed these together and the heads appears to be unloading ok.
After the motor cuts out I can hear the hissing. I also unconnected
the lines to ensure no air was trapped in the heads. There was none.
However it still does not want to restart even at a tank pressure of 70
lbs. I can rotate the pump by hand at the end of the unloading so I am
pretty sure it is completely unloaded.

During its initial run from an empty tank I was reading about 37 amps
on one leg from my 10 hp phase converter near the high pressure end
~120 psi before shutting off. The motor is a continuous duty 230v 3
phase rated at 27 amps. So I am not sure why it is reading so high. I
believe these motors are suppose to run warm. Mine does, I wonder if it
is heating up too much.

So my next step is to find a bigger converter. I have a surplus 15 hp
motor but I can not start it even with my 10 hp converter and using a
pony motor to bring up the speed. I checked all 3 legs with a DC
battery and the shaft was resistive. I have read in these groups that
I should be able to start that unloaded motor with my phase converter.
This motor is a Vibration rated induction motor and is a beast - about
300 lbs.