As far as the re-charge stuff goes: that works ok if the system is just low and needs to be topped off. In that case there is still system pressure and no air/moisture got in. But even here the gunk that the compressor makes (it's like a sludge, metall wear in the pistons and drive of the compressor) still collects and with age that will also affect the system... That's not to say you can't touch it, it's just hard (if not impossible) to do this right without specific tools...