2000 Isuzu NPR WEIRD A/C PRESSURE READINGS

VIN #4KLC4B1R6YJ805937

Isuzu NPR with a mounted box food delivery body. Driver said A/C just stopped working Monday when he got in the truck; I test drove the truck Saturday after replacing the rear brakes and the ac worked fine. I hooked up the gauges and had no low side pressure and 50 psi high side. I checked the low pressure switch on the dryer and it had power so I installed a paper clip so the compressor would engage. I put a can of 134a leak detector in the system and the pressures started going up slowly but then started back down; the low pressure ended up going in a vacuum. I checked for leaks and couldn't find any so I started adding 134a a little at a time to see if the pressures would go back up. After a couple minutes they did and the vent temp went from approx. 100 deg to around 75 deg; I reconnected the low pressure switch plug and the compressor worked on system pressures. I could feel the system cooling but then the compressor popoff valve started venting even though the high pressure was only around 100-120psi. I stopped adding freon and checked the system; the compressor started cycling, the low pressure was in vacuum and the high pressure was between 50-70 psi. I'm guessing expansion valve but not sure, have never had a system go into vacuum before. Need an answer asap, thanks.