'03 Ram 3500 A/C problems

Bought a used 105,000 mile Ram 3500 Cummins. Seller gave me all the service records/etc. A/C was 'fixed' to the tune of $1409.00. Invoice showed new evaporator and a line. The work was done at a Dodge dealer in January. I bought the truck in Feb. 2days later,,NO A/C. NO help from the dealer after the seller said he would contact them. Took truck to independent shop and they said no Freon in system, all leaked out. Vacuumed/charged system w/dye. 5 days later, no A/C. Took back and they put the light to the underhood area,,no leaks found. Must be in the dash,,took dash apart and no dye/leaks found. The evaporator was not a new one. I did have them put a new heater coil in while they were there, so it was not all labor time lost. Vacuumed/charged again,,no air 5 days later. Anybody got any ideas?? I would think that big of leak would be easily found. Condensor has been inspected,,the dealer did put a new accumulator/drier on it and charged for an evaporator.

I've seen the service port Schrader valves leak. Take the caps off and put small balloons over them. If the balloons inflate the port leaks.

Easy to change but you'll need to evacuate and recharge.