POA valves are typically found...
POA valves are typically found on '65-73 GM vehicles as well as '70-80 Fords. Activated by pressure they don't work properly with R-134a owing to higher system pressures associated with the newer refrigerant.
Rick Love of Vintage Air told us that the molecules in R-134a are smaller than those of its predecessor, and as such over time may leak from old-style flare fittings, which is why new systems employ O-rings. He also mentioned that the pressure/temperature curve is different between the refrigerants, and with GM products from the Sixties in particular, a temperature cycling switch may be required to replace the POA valve (Pilot Operated Valve-replaced by a Valve In Receiver valve, or VIR, in '73), which keeps the evaporator coil from freezing. Also, if an expansion valve is part of the system, a new one recalibrated to work with R-134a will be needed too. Of course Vintage Air manufactures new A/C systems, so their interest lies in replacing rather than retrofitting, but Rick knows many will want to keep the original system and has even retrofitted a few himself in the past.
Shane at Southern Air reiterated Rick Love's comments about swapping out the compressor, condenser, drier and hoses, saying many of their customers with older cars fit a new system using the factory evaporator and controls, and disguise the converted controls with a new fascia plate in the interior. This is something we'll be doing in our project '49, using the factory heat and air controls to operate the new A/C system from Vintage Air.
But we're discussing retrofitting here, so how's it done? As a general rule of thumb, the old mineral oil has to be removed from the system, the accumulator or drier should be replaced with one containing X-7 desiccant, a new orifice tube or recalibrated expansion valve will need to be fitted as will a high pressure cutout switch, and the O-rings will have to be replaced. A more efficient condenser may also be needed. The hoses should also be changed to a barrier hose, which contains a nitrile rubber inner tube and neoprene cover that prevents water from entering or refrigerant from leaking through the actual hose walls. Then PAG oil can be added and the system recharged with R-134a to around 80 percent of its original capacity. You're also required by Federal law to install R-134a fittings on the high and low service ports which will identify the system as containing that refrigerant and reduce contamination in the future.
Clint Millican at Old Air Products told us that while much retrofitting is model-specific, all will need, as a minimum, a new receiver/dryer, oil change and system flush, as well as new service parts according to the new refrigerant. Other additions will then depend on the vehicle. For instance '59-61 GM vehicles use a hot gas bypass valve that contains rubber parts and diaphragms that are not compatible with R-134a, and as such should continue to use R-12 if originality is required, while from '62 to '65 the STV (Suction throttling valve) should be updated to a CCOT, or cycling clutch valve, as the higher pressures at which R-134a works will force the STV to close before the temperature drops to where it should be. Old Air Products also offers an eliminator kit to convert '73-76 GM VIR assemblies to an orifice tube. Variable orifice tubes allow the flow rate to change to provide improved cooling at low speeds and in stationary traffic.
If you're converting a GM, Ford or Chrysler A/C system from the Sixties, however, you may experience slightly higher temperatures with the A/C running after switching to R-134a because these systems are controlled by the low side pressure, and R-134a produces lower pressure at a given temperature than R-12.