Never just keep topping up an E36 A/C that's lost its charge — an empty system has leaked, so find it first.
The easiest DIY method is fluorescent UV dye: add dye, run the A/C, then shine a UV light over the fittings, condenser, compressor shaft and evaporator drain and look for the glow. An electronic leak detector, or soapy-water bubbles on a slightly-charged system, also work
…