I have a 69 Camaro, factory air with the following changes. Old Air POA update, larger more efficient condenser and a Sanden 508 compressor using R134 Freon. The gauge readings (with an ambient temperature of 83 degrees and unknown high humidity) were 24-42/170-180 with 48 oz.(4 12oz. cans) of freon and the engine running at 1100 RPM with a stationary cooling fan in front of the condenser. The problem is that before the low side pressure switch (on the POA update) turns off the compressor (due to low pressure...24 lbs.) the A/C cools very well but, as the low side pressure rises, so does the cabin temperature. After adding 36oz. (3 cans) I thought it was acting this way due to low freon so I added the other 12 oz. without much change except for a slight rise in the high side. I'm trying to figure out why this is happening and if I can get the low side to run at a more constant range so the pressure switch won't cut of the compressor as frequently. The pressure switch is adjustable but I fear going too low, freezing up the evaporator, which would starve the compressor of oil. Does anyone know what my problem might be or how I could find out? Thanks

I have never used the old poa valve set up but I do work with 134 and I have found that filling the unit up within 80% of what R12 volume was, helps even thought you may have to get a higher hi side pressure and little higher low side.
The other method of testing is to get the low side (cold line) inlet and outlet , within 5 degrees of each other, then leave it alone.
How about the dryer? Got a sight glass? I dont think you will freeze unit up unless you dont have enuff temp flow across evap.

Does that mean that if the inlet temp is colder than the outlet, it's undercharged? If so, what type thermometer would you use to measure it? Thanks