I replaced the suspect compressor tonight with a known-good used compressor. That was the problem. As soon as the compressor clutch kicked in, the low pressure went down and the high pressure went up to the expected values with steady gauge readings. I figure that the suspect compressor had one or more bad reed valves.

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)