First, turn the plate in the front of the clutch and make sure the compressor is free. If it won't turn by hand, or if it is rough or notchy...it's done. If it turns by hand and you can feel the compression, then you're in business. Next, see if the system can hold a vacuum, as you'll want to repair any leaks beforehand. If it passes those two tests - and barring any other issues - then it's likely a viable candidate for the conversion to R134. Yes, to do it right, you'll want to get out as much of the old R12 oil out as you can, but you'll never get it all out. Some of it can be recovered during the evacuation process, and some you can remove manually. The two items that hold the most oil are the compressor and the dryer. I would definitely replace the dryer anyway. If you want to empty the compressor, the easiest way is to remove the drain plug and invert it. Shop air & brake cleaner work well with pipes & hoses. I would not shoot any into the condenser or evaporator though. Just blow through them with shop air (or remove and drain) as any residual cleaner will not mix well with the new R134 oil, and could cause future problems. Be sure to wear a mask and get in a well-ventilated area. Don't worry if you don't remove all of the old R12 oil, as it won't mix with R134 oil. In other words, after the conversion, the old R12 oil will tend to settle in the low areas of the system and stay there. Having said all of this, I've seen guys do nothing more than buy the kit, and shoot it in there... It's up to you how involved you want to get. Considering you only want it to work temporarily, maybe that would do just fine. If you want to do it yourself, the conversion kit can be purchased at any auto parts store. Good luck with it!
