Based on the troubleshooting, it sounds like I may be low on refrigerant. I don't know a lot (read: hardly anything at all) about air conditioning. How can I check to see if the old system needs a recharge? I last had it serviced in 2006, so I had a good run. . .

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1982 240D - 292,000 miles and counting
Same family for 33 years
Newly rebuilt Monark injectors
Newly replaced clutch
New Bilstein shocks