Of course the easiest cheapest possibilities are not too obvious...
What I am referring to are the service port valves (where you hook up the charging hoses.)
These ports have little Schrader valves in them, much like a tire valve.
Over the decades, the rubber seals in them dry out and shrink and deteriorate, causing leaks.

Upon restoring the A/C hoses as a part of my engine compartment restoration (https://classicoldsmobile.com/forums...o-unknown.html), I decided to check those valves...

These can be removed with a valve core removal tool - available anywhere auto or bicycle parts are sold.
The high side valve was just BARELY snug. The low side valve was loose - as in no snugness at all. In fact I thought my tool was not even on anything.
Upon removing them, the black seals were dry, hard, and breaking. The sealing surface had pits in them, too.
Heres what they look like:



So I am convinced these were a cause of my major leaks. I am thinking that if I had snugged these up when I had just bought the car, I would have still had A/C for a little while. I do know the evap core was leaking, too, just because oil was dripping from it.

I will stop by the local A/C shop for some new valve cores. I doubt tire valves would be appropriate in an A/C system.

 

Rob,

I just tried adding a can of that freon you can get at the parts stores and it leaked right back out because my valve was either loose or shot like yours. A couple days later, I happened to see a package at O'Rielly's that had replacement shrader valves in it so you might check there if your local a/c place is real expensive on them. They were hanging near the freon so I'm pretty sure that's what there were for.
I'm wondering though, since you'll be opening the system up to outside air when you take those out, won't you(we) have to have the system evacuated before adding freon again?

 

A/C valve cores are made to handle higher pressure.

W70442... yes and no on the question. if you recover the refrigerant to a very slight positive pressure, like 1 to 2 lbs positive pressure. you could take one out and another in without any moisture or outside air getting in. then you could just recharge it. due to the EPA that is illegal to do it that way but it is the easiest. if you don't have experience with this stuff i would recommend just evacuating it to be safe.

 

Try tightening the valve core first, especially if the system still had some pressure.

A general rule of thumb is that if the system has no pressure, then it needs evacuation. Basically, you need to suck any air out to make room for Freon. Removing the air also removes harmful moisture.
It is possible you could get away with using your own vacuum pump to do this.

Nice to know the valves are widely available. I thought they would be hard to find.
Now I wonder if they are different for R12 and 134 ports?