If refrigerant leaks
out fast when box is warm and much slower when box is cold leak is in low pressure side of system. When Box is cold low refrigerant pressure is 2 to 7 psi and less likely to leak out. I would add refrigerant when box is warm and with compressor
off check for leak, at this point pressure in complete system will be 30 to 70 psi depending on temperature of complete system. Places to test first; Service
port with cap installed, Line connector between evaporator and compressor
, and complete evaporator.