A/C

Well, it may not be the o-rings on the compressor that are leaking after all. When the car was converted to use 134a, the kit they used had a high pressure adapter that has a 90° swivel bend in it. I was checking to see if it was loose and it turns out it was. I started to tighten it down but then though I'd better check the o-ring in it... voila! The o-ring is toast.
Closer inspection reveals that this o-ring is even more critical than usual on an adapter. The adapters I've seen are straight and very short. They do not have their own schrader valve but rely on the original R-12 valve. The o-ring in these adapters is only to seal when charging the system. As soon as the hose set is removed from the adapter, the original core seals shut and the o-ring does nothing, essentially. But because this adapter has the bend, the original core was removed and there is a schrader valve in the adapter instead. The o-ring now has to hold pressure all of the time, not just when charging. And with the swivel, it has to be a perfect seal or it leaks like crazy. Copper flare-nut type swivels, which this is similar to, can be really tightened down to seal and the soft copper will crimp itself to the shape needed to seal as you go tighter. But you can only tighten this swivel down so far before the o-ring distorts and tears... exactly what happened.

Off to Pep Boys for a replacement high side adapter... of course they only had the pair in a full conversion kit. No matter, it included a new schrader valve core, which I was going to need as mine had been removed. The bend in the old adapter had made it easier to connect the charging hose, but I found that it wasn't much more trouble to connect with the straight stubby one though. The seal will be so much more reliable it will be well worth any minor inconvenience anyway.

Right now I have a 15 in vacuum pulled on the system and I'm waiting to see if it holds. I had already ordered an assortment of o-rings and if the system won't hold the vacuum I'll need to wait for them to arrive before I can charge. But I think these o-rings are OK. For one thing, the compressor is a rebuilt unit and the rings are green, so they are correct for 134a. They can't be that old!