I couldn't find any specs on low, mid, and high readings, so I'd suggest testing the sender resistance with engine off, idling and higher rpm and compare readings. I'll try and come up with some numbers for you.
with the ignition off, resistance from the sending unit to ground should be 280 to 300 ohms
with the engine idling, resistance from the sending unit to ground should be 150 to 180 ohms
if your sending unit is within range, you'll have to replace the gauge
I don't know what I did, but after I tested it and messed with it for a while, it registered pressure again. Probably needs to be replaced, but it will work for now.