Don't know what to tell you about the leak warning. We had to replace our
760 compressor and had Volvo do the 134a retrofit nearly ten years ago, and
it hasn't had to be recharged since. But since yours stopped cooling, and
you didn't indicate a repair with the conversion, you must have a little
leak (too small to prevent the conversion.)

Time will tell how significant your leak is, and if it gets worse. Next
summer, if it doesn't cool well, time for service again. Should last all
season, anyway. (And that should be pretty much independent of how much you
use it, like a tire leak behaves.)

Mike

It should last years if not decades. Did you have all the O-rings and
rubber hoses replaced? If your system was dead before you had it
converted it's because it leaked somewhere.