I'm just wondering how much longer you plan on having the jeep. Technically Bill and Marty are right; you should always vacuum out the system after servicing it. That said, I install a lot of those r134a "Conversion" kits on old R12 systems. Most of them have been empty for as long as the owner remembers, or I need to pull the system apart and replace o-rings. They are all on higher mileage vehicles (my own included) and I never evacuate the system after I've had it open. I just don't have the tools, and I don't wanna spend more than I have to. Now my neighbors 89 XJ has been running the wrong refrigerant for almost 7 years in a system that had a bad hose and empty/open system... If you don't plan on owning your jeep for another decade, I'd save the money.