if theres no gas in the system then theres probably moisture in the pipework, to be honest id take it to the pro's. im an a/c engineer and i wouldnt do my car myself as its a very expensive mistake if overcharged or you have moisture in the system. potentially a new air con compressor required. saying that ive had no issues with mines and when the time comes i probably will give it a bash, but there is auto a/c engineers out there that know what there doing and paid well for a reason