no high pressure rv2 help!

There was no EPR (Evaporator Pressure Regulator) valve used on A-body A/C systems until 1974. Instead, the pre-'74 systems used a clutch cycling thermal switch with its temperature probe inserted between fins of the evaporator. On systems through '72, the setpoint of this switch is adjustable by operating the cool/hot slide lever on the dashboard. For '73, a nonadjustable switch is used in conjunction with a blend-air system (operating the cool/hot slide lever blends hot air from the heater core with cold air from the evaporator). The clutch cycling switch opens or completes the circuit to the compressor clutch so as to maintain the setpoint. The minimum setpoint is a degree or two or three above 32°F; the point is to maximise cooling but prevent the evaporator from freezing. If the evaporator drops to 32°F or below for more than a brief few moments, condensed water will freeze and block airflow through the evaporator.

The EPR valve accomplishes the same task (preventing the evaporator freezing) by selectively throttling the low side of the system. On a system with an EPR valve, the compressor clutch has power whenever an A/C mode is selected, unless the high- or low-pressure safety cutout switch opens the circuit.

So, to review, there should not be an EPR valve in the low side (rear) port of your compressor on your pre-1974 A-body. If there is one, it won't do any harm as long as it's working correctly, but if it is not, it'll bollix things up.