If I remember my GM history correctly cycling clutch started in 77. So your comp should stay engaged unless low or high pressure switches open as protection. So far so good. Lack of differential pressure points to the valve. They changed types and name for expansion valve or POA over the years. Obviously adding refigerant will raise the pressures low and high side. How does the low side site glass look as you charge it? Can you see vapor rushing past it? If so I would guess comp is ok. I think Emery said the glass is not accurate as a charge guide with HC12. If it goes clear liquid it is overcharged, unlike w R12. Sounds like you never reach a good point as you added and monitored charge so charge amount would have been good and cold at some point. I would refer to the FSM for your year to understand and diagnose your year's valve system. I think there is a bleed tube and temp sensing to control the throttling of the valve. In other words it's an active valving component, unlike the fixed orifice size and temp control done by cycling the comp on the later years. I'm guessing there is not enough pressure building across the valve. You should hear an RPM drop as you cycle from vent to AC, if only slight then the pump isn't using any horsepower to do work against the restriction. Do Jim Ks techs have any insights using this comp on your year with HC12?