Well, as it happens Volvo recognized that all 740 and 780 cars and the 1991 940 have this problem: no way to limit a/c system high pressures. In the Green Book for HVAC, page 334, there is a modification to add just such a switch by replacing a liquid line and adding the switch (Volvo p/n 3522247-0) and wiring harness (p/n 3522545-7). The new liquid lines vary depending on engine and model. I wish I had known about this earlier: I would have done it years ago.
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See the 700/900 FAQ at the drop-down menu above right.