Having trouble finding AC leak

Check the charge port valve cores
+1 Usually the first place to check, and probably the most common leaking point. People often get fooled because after pulling vacuum and leaving the gauges hooked up to see if the system holds vacuum, a Schrader valve that's leaking will not be detected with the gauges hooked up. That was a lesson I learned the hard way.
 
The caps on the service ports should hold the refrigerant in even if the schrader valve leaks. Although if you have the cheap tall aluminum Interdynamics R-134a adapter on the low side, it will leak where it connects to the R-12 fitting and ought to be replaced with a steel one.

First place I would check on a 1990s car would be the connections between the compressor and the condenser. The refrigerant runs hot there and the heat will harden the o-rings and cause leaks.
 
Turns out that little cheapo amazon leak detector has given me a clue on this rabbit trail. I charged the A/C just enough to get the compressor to kick on. The leak detector detected no refrigerant near the valves, nothing at the compressor, or drier. I was doubtful the thing was effective. I ran the thing in the cabin, and then it started clicking like mad. I positioned the detector at the vents, and it signaled a consistent and traceable leak. I am surprised that a cheapo detector like this could be so helpful. I am taking the Van to the mechanic tomorrow to extract all the AC. I will take the evaporator out and inspect it for leaks. Hopefully I can trace down the leak and eliminate it.

Is it worth it to replace other seals in the AC system? Ebay has cheap compressor shaft seal and o-rings. Are things like that worth replacing while the AC system is empty? I DO plan to keep this van a long time, since they are known to run forever. AC is one luxury I can't live without. I just don't know if the oem seals are better than cheapo replacements.
 
On an vehicle that old, I would not go replacing compressor seals or even o-rings unless they were actually leaking. At this time, R-134a Freon is cheap, so I would stock up in case the EPA decides to ban it like R-12 & R-22. With the sniffer, you should be able to locate any future leaks easily, should they occur. I had a dark green 1995 Previa LE for 14 years and 269K miles and the A/C never leaked or needed a charge!
 
If evaporator is leaking you should also have a strong hit at the drain hose (with car off), as refrigerant is heavier than air.

Replace o-rings in any connection you take apart and the high side hose that I mentioned. Otherwise the more stuff you disturb, the more likely to cause an additional leak. I definitely wouldn't mess with the shaft seal unless leaking.
 
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