It could be a bad low pressure switch. Unfortunately you have to evacuate the system to replace it. On a car with over 300K miles, I'd be tempted to jump the low pressure switch, with full disclosure to the buyer of course, and enjoy the summer. I've never heard of a high pressure failure in these cars though you might have tilted the table with the extra 12oz Cooling capability will be notably diminished before the low pressure trigger so you'd know to do something about it if it fails in the summer.

Sixto
87 300D