If it loses pressure, it has a leak, although a small one.

Most likely you have leaky o-rings or a compressor seal, these are the normal problems. The expansion valve will also leak, but that is usually somewhat faster.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!