Just got a 93 sierra . 4.3L with 5 speed. Like the truck so far. But the AC doesnt work. The PO suggested recharging the system. Then I realized it was r-12. I would like to just recharge the system with r-12 and see how it works. But it could be compromised in some way. Please advise...and if you think a retrofit is in order. What all needs to be done for a retro...???