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My first suspect would be a bad compressor shaft seal. Refrigerant and oil circulate through the whole system. When the unit is used every day, the compressor shaft seal and any tiny leaks at "O" rings and crimped hose fittings have a coating of oil on them, which tends to slow down the leak. When they sit unused, these areas will tend to leak more. Almost always, there will be an oily deposit, with dirt stuck to it, if it is a fitting or line leak. A bad compressor seal will throw oil out ahead of the belt pulley, though the plate on the magnetic clutch. On the "40" series, there are hydraulic oil cooler hoses that dangle ahead of the AC condensor, and sometimes wear though the condensor and create at tiny leak (at first). I don't know if that is a possibility on a "50" series. The refrigerant lines from the cab top drop down through the rear LH cab corner. They are insulated, which traps moisture and dirt, and they sometimes get pinholes rusted in them. Beg, borrow, buy or steal a refrigerant leak detector. Allow the tractor to sit a half-day. or so, inside, out of the wind. Then, check for leaks at the compressor clutch, all fittings, and at the bottom of the cab corner, where the lines exit. Refrigerant is heavier than air, and tends to fall, so check UNDER areas that may have a leak. When you've checked out everything else, put the sniffer tip next to the air vents in the cab top. If there is a leak up in the evaporator box up on top, the sniffer should detect refrigerant dropping out the vents. You can turn the fans on "LOW", and click the key on for a second, to blow a shot of air out the vents, and see if the detector picks up anything! Good Luck, likely, your gonna need it!
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