Best way to test is to pressure test with a nitrogen/hydrogen mix. They will charge the system at about 200 psi and then use a sniffer which will detect the hydrogen leaking from any seal or fitting. Sniffer will indicate the level of leak in a gram or ounce per year scale. Vacuum alone will not be accurate, shop air is just adding moisture and there is no way to sniff for leaks. If you want to keep R12 or not waste R134 which is getting more expensive, find a shop that has this setup. Save yourself future depression and find and fix the leaks now.