71 K20 AC servicing info with POA valve

[hydrospec2001]

Guys I’m hoping somebody can help ma make sense of servicing my system on my 71. It had an oversized parallel flow condenser , Sanden 709 compressor new drier and expansion valve. The POA valve is adjusted to 23 so I can run R152A as I was told it could run colder than R134. I live in Arizona so I will do all I can to get the temps down in the cab. The questions I have are service related.
When I service the system it takes approximately 2 10oz cans of R152a before the POA valve will open and stop the line from frosting. My high pressure is around 160 and the low side is at 23. Vent temperature is around 50. Outside temperature this morning was 93. If I rev up the motor the low side pressure will drop to 10-15 and the line started to frost. I started to add some more refrigerant. I stopped when the low pressure was around 25psi. I went for a drive and as I would slow to a stop I could hear the POA valve groan as if it was allowing circulation through the evaporator again. It appears it lost the pressure to overcome the poa valve. Am I still low on refrigerant? Should I be servicing the truck at a speed higher than idle such as 1500 rpm so the valve will open under load while driving? I really hope this makes sense. I’m not much on typing and better at talking but I’ve looked about servicing on here and even other sites with vehicles that use a poa valve but can’t seem to find servicing information. I’ve serviced multiple systems but they were all cycling switch systems so I’m just trying to find clarification. Any help would be great! Thanks!

[Richard]

I am not familiar with R152A. I can see one thing that is overlooked in condenser mounting. Stock type with condenser away from rad allows to much airflow around the condenser at slow speeds or stationary. 134A and others require more heat rejection, putting the condenser against rad or creating a shroud from condenser that seals to rad will improve airflow in those conditions.

[truckdude239]

R152a is classified as a non-toxic but mildly flammable Class A2 refrigerant – medium
safety level – by ASHRAE, not sure id be running a even mildly flammable refrigerant