I finally got a low pressure switch. It says:
"HFC-134a
HP: OFF 3.1 Mpa
LP: OFF 0.2 Mpa"
I figure "On" between around 30psi through about 400psi.
It screws into the receiver/drier.
I already have a relay for the fan but it takes power from the glow plugs.
This is the final deal as long as nothing leaks:
That's the switch on the receiver.