A 405 Hunter still has the old R12 system which means two things.
1) the high-side and low-side valves are mounted on the end of the compressor, right up close to the sump, covered in sludge and almost inacessable.
2) you can't just have it "topped up" If it's still running with R12 it will need a conversion to R134a, or a Drop-in of R413a. (details are all over the aircon forum)