No A/C experience?

if you disconnect the lines, you'll vent the refrigerant. Not good but it does it by itself millions of times a year across America via slow leaks. Once the dry R12/R134 (what year is it, anything after 1993 is probably R134) has degassed, ambient moisture from the air will creep in and saturate the desiccant in the drier. That's why you want to seal it up. If you plan on getting a new drier, they are usually pretty cheap. Also R134a refrigerants produce an acid when exposed to moisture so you want to flush that old stuff out ASAP. You must determine what oil to use for R134. R12 Mineral wont work. POE is universal and PAG is R134a only. Once you get the system back together, you can add your oil then fill with an inert gas to test for leaks with soapy water at joints and condensor. Once its determined to be airtight, vent and pull a vacuum for 30 minutes and then introduce the required weight of refrigerant. It will be on the A/C sticker under the hood if its still intact. Use of a gauge manifold makes this process easy as everything is hooked up at once (vacuum, freon, low side fitting) and is isolated via the valves. You can use a refrigerator pumps inlet as a poor mans vacuum pump. It will pull down to almost 27.0 and @ 100F (running engine bay) thats enough to boil moisture out. let it run for a while. Tighten the valve for the vacuum source and turn off pump. Now you got an evacuated system, ready for the charge. Hook up the refrigerant to the other valve side, and burp the line to make sure there is no air in it. Once everything is hooked up, start the motor and turn the A/C on full. Open the freon side valve and it will start to flow into the system. The compressor will not kick in until there is enough pressure in the low side to kick the safety switch, then it will start to cycle the A/C clutch. Use a scale on the freon tank to get the proper charge. you can use the low side guage but its better to use a scale to get the proper charge. An A/C guy can chime in and flame my process on this but this is how I have done numerous cars. See my post on Propane as a refrigerant if your uber cheap and your **** has a slow leak anyway and you must pay $20 for 12oz of R134, welcome to California. Now in Texas, where its $3.99 a can, go for it.