Factory A/C in 1967-1972 Darts

Jim Kueneman said

My 72 Duster with R134a (with a retro fitted 73 Dart AC box) works poorly. I have spent a fortune trying to understand why. I recently discovered that the 73 AC box is less than a one year design. It was changed again in 74 and stayed till 75 and the end of these A bodies.

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'73 still used a clutch-cycling switch like the '65-'72 systems, adjusted via the warm-cool slider on the dash, to regulate the evaporator temperature, and did not route defogger air through the evaporator to dehumidify it for much better defogger performance. For '74 the A-body got an EPR valve (evaporator pressure regulator, which is what Chrysler called a suction-throttling valve) installed in the intake port of the compressor, just like the bigger cars had been getting since '62; the clutch-cycling switch was deleted, and defog air was routed through the evaporator.

You're right that an R134a swap does not necessarily require new hoses or a new compressor shaft seal, but the EPR valve's calibration is specific to R12, and can really take a bite out of system performance under high demand when R134a is used instead. The best way to avoid this is to delete the EPR valve and install a thermo-sensing clutch cutout switch. It's like the clutch-cycling switch, but not driver-adjustable—you install it so as to stop the compressor when the evaporator gets cold enough to ice up. This technique also affords you some extra system performance if you live in a dry climate where evaporator frost-up is not such a pressing concern; you can install the switch so as to let the evaporator get a little colder than you'd want to allow in a more humid clime.

An R134a changeover practically it really does want a better (parallel-flow) condenser, though. Swap one in and watch your numbers on the manifold gauge get much better.