Dick. During assembly of my 66 I had to make the same decision. This is what I think I have read in the meantime.
1) The PAO valve regulates the pressure in the evaporator, making it independent from outside temperatures and altitude. The compressor is always on and the POA thus regulates.
Because of the different pressures with R134A the POA valve should theoretically be replaced. Some people say they didn't and their A/C is working just fine. I have no exprerience of my own.
Old Air Products sells an 'update' that replaces the POA valve. I had bought the update and found that it is nothing more that a tube (with the right threaded ends) that mounts in place of the POA. In the tube there is an adjustable pressure switch that must be used to cycle the compressor. In other words the POA regulation is removed and the compressor will be used to regulate things instead.
(Because one of the threads was bad, the supplier sent me a replacement. Living in the Netherlands this took me too long so I used my old POA valve and hope to be able to get some R12 later on.)
2) The cooling capacity of R134 is somewhat different resulting in higher condenser temperatures. In modern systems the condenser is bigger because of that.
3) Theoretically you should change all of the rubber parts when converting from R12 to R134A. Many shops say only the drier and the oil need replacing.
4) Some people say when mixing the 2 types (converting from R12 to R134 without a thorough flush) the 2 will interfere resulting in clogging.
5) Esther oil is said to be usable in both systems.
6) The R134A is less dense causing it to leak out sooner when rubber parts are porous.
Hope this helps a little.
Rob
