Get some a/c oil (PAG/ester) that has dye in it & inject about an ounce into the system. Then, while you drive from place to place doing what ever. Just keep the a/c on. After some time, get a black light, yellow glasses, & see if you can see any dye traces along the whole a/c system. Pipes, compressor, dryer, condenser, even at the drain for the evap core. If you can't see the drain, check the water that comes out from it. It should have traces of the oil/dye in it.
If you see no traces at all. Then it may be the schrader valves. You can always test this first by pouring a little bit of a/c oil in them & see if you see any bubbles come up. Give it some time, cause it can be a small leak.