Thanks everyone

I do have the ubiquitous Harbor Freight Gauges. Thanks Bill for clarifying how those ones work.

I think the high side valve core must be stuck.

New problem. I went to go take the gauge set off, and the low side service port just spit refrigerant at me. I quickly put the gauge set back on.

This must mean I have a bad valve core on the low side? Please tell me there's some way to fix this without removing all the refrigerant. That hose is brand new, do they not come with valve cores? I remember checking for one and seeing it.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car