The PBC if ok, should not stop the normal operation of the a/c system unless the R134 is so low that the compressor gets locked out. But it has to be very low for that to happen; if you down even 1 lb (in a 2.5 lb system), it will still work.
I had what I was sure was a bad PBC! But it turned out to be a siezed compressor. The clue was reading the DTCs in the BASE MODULE (my page, MENU#8); sure enough there was a "DTC 7; belt slipping" code stored!
Anyway, good hunting!
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page