A/C not working
 

1985 911 A/C not working. Checked the hi and low side pressures and when I first started the engine and turned on the compressor I had apprx. 40 psi on the high side and 0 on the low side. right after the compressor ran a couple of seconds the high side went to 0 and the low side was reading a neg pressure. I noticed this model has a expansion valve on it. are these valves prone to failure? Also this car is still using r-12 can this car be converted to R=134A with the standard conversion kits from the local parts supply....

You need to find out where your leak problem is before you put a lot of coolant in it. You can put 134a in but you will need to flush the system since the oil used w/R12 doesn't mix with 134A. The factory hoses for that year are designed to leak very slowly. You would probably need to replace all the hoses (no small project).
Do a search. There is a ton of info already on here about your situation.