In sunny California Air Conditioning is a must... and so it happened that in December 1998 the AC gave out immediately after letting the Dealer fix an electrical problem. I took the car to a local repair shop an told him that the AC was empty. He filled it with R12, found no leaks and sent me on my way less $100. Three months later it was empty again and I took it back. After much fiddling, he informed me that the compressor had a leak and needed replacing. He assured me that he was competent at repairing AC systems and that in addition to the compressor that I should replace the dryer canister and the oriface filter as well as flushing the system and converting to HFC134a from CFC R12. I agreed and he began work. Immediately his incompetence began to show. He promptly destroyed the high pressure coil line trying to remove the filter and had to order all sorts of adapters to reconstruct the system. $600 later he gave me the car back declaring it was a finished work of art. One week later the freon had disappeared into the ozone and I returned the car to him. No less than 6 times did I return the car to him because every week the freon was gone. Finally enough was enough and I decided to fix it myself. BTW the shop is called Del Mar Auto in Pasadena, CA.
I purchased a fill kit with Dial indicator and fill lines, a couple of cans of stop leak/detector and some freon. Kits such as these from Interdynamics are readily available at most auto stores or through JC Whitney. I promptly added the stop leak and then filled it with freon. Several weeks went by and AC still worked. I bought a small can of O-ring/compressor oil and added it to the mix. The car sat for two days and upon trying the AC, it was empty. I now had a clue. Upon filling the AC with oil, I noticed that a small amount of fluid was bubbling out the low pressure port when I was done but I hadn't thought much of it. Evidently one of two things was happening. Either the valves randomly don't seat properly, or the oil interferes with valve seating. I refilled with freon (fortunately 134a is cheap) after replacing the valve cores and installing a 90 degree adapter with it's own valve core and housing. Time will tell if this solves the problem. I have heard from others that the fill valves are notorious for leaks.
Things to remember when filling your AC (not explained in the instructions), first the stop leak if needed, then the oil charge and then the freon to clear the ports out. Make sure there isn't even the slightest sign of leakage.