I searched and found a great write-up Les did earlier. Here it is.
1) If the R-12 vehicle air conditioning system is operational, run it at idle with the A/C blower on high speed for five (5) minutes to optimize the amount of oil in the compressor.
2) Recover all R-12 refrigerant from the vehicle's A/C system. Evacuate the A/C system for at least thirty (30) minutes to a vacuum of 29 in. Hg, using R-12 equipment, to remove as much R-12 as possible from the residual mineral oil. Don't just dump the old R12 into the air. If you really don't want it, at least let a service station vacuum it out. They will generally take it very happily, as it is quite expensive, and you won't be polluting.
3) Remove the compressor from the vehicle, noting the placement of all shims and washers, and the routing of hoses and wires.
4) Remove the compressor oil plug and then drain as much mineral oil as possible from the compressor body.
5) Drain mineral oil from the cylinder head suction and discharge ports while turning the shaft with a socket wrench on the clutch armature retaining nut.
6) Remove the existing R-12 receiver-drier or accumulator-drier from the vehicle and discard. Allow as much oil as possible to drain from the A/C hoses. Blowing them out with an air gun on moderate pressure is okay. Back-flushing the lines is recommended. Plain old mineral spirits swished around in the compressor and backflushed through the lines will remove contaminants and old mineral oil. Make DARN, DARN sure that you get ALL of it out of the system, and I do mean ALL of it. Mineral spirits can be explosive, but a great many a/c shops still use this technique to great effect. I would recommend drying everything out with clean dry air from a compressor for quite a while (hey, air is free, right?) Commercial a/c flushing chemicals are also available which will do the same while being safer, but they can be pricey.
6) Replace any O-rings on the receiver-drier or accumulator-drier joints; check and replace other O-rings that have been disturbed. You really should replace them with the new green ones -- they work very well for both R12 and R134a. If you are converting to R134, new reports suggest that you do -not- need to change all of the O-rings to different materials as thought earlier. However, I would recommend changing anywhere you have disconnected a joint, just to help seal better. They're cheap anyway, and cheaper than having to do it again because of a leaky seal.
7) Replace the accumulator-drier with a new R134a compatible unit which contains XH7 or XH9 desiccant. Make sure that you get one with "XH7" or "XH9" dessicant, not "XH5" -- there should be a sticker on it stating as such. XH7/9 works just fine with R12 and is necessary with R134's Ester or PAG oils. As a cross-reference, a 1986 Corvette accumulator-drier is perfect. Factory Air brand part number 33191 dryer is ideal, and runs about $55 at AutoZone.
8) While you have the accumulator-drier removed, now is a good time to replace the orifice tube. Always replace it 'just because'. The type used on the DMC is just a standard "white" GM orifice tube used for many years on many GM models, and the cost is less than $2. (Any parts counter person should know what you mean when you ask for one). There are some newer variable orifice tubes claiming to work better with R134a, but I have not read very much about them, and I'm a little wary of their claims.
9) Perform any necessary repairs to the compressor or A/C system.
10) Using the original refrigerant oil quantity specification, add [INSERT QUANTITY HERE] ounces of Ester oil to the compressor. Ester oil is preferable to PAG in a retrofit, as it will mix fairly well with any remaining mineral oil. It was originally thought this was not the case, but consensus nowadays is that they mix satisfactorily. Ester is also less corrosive than PAG. As an additional benefit, R-12 will also work with Ester should you ever wish to convert back. If you follow these guidlines, all you would need to do to revert to R-12 is completely vacuum all R134 out of the system and then simply reinstall R-12.
11) Replace the compressor oil plug O-ring with an new O-ring.
12) Reinstall the compressor oil plug. The plug seat and O-ring must be clean and free of damage. Torque the plug to 11-18 ft lb (15-25 N m, 150-250 kgf cm).
13) Change any seals at the compressor ports to new seals.
14) Reinstall the compressor to the A/C system, paying close attention to the placement of shims and washers from step #3.
15) Disable the R-12 service fittings to prevent any refrigerant other than R134a from being used. You do this by permanently installing R134a quick-connect service fittings to the A/C system.
16) Vacuum the system for AT LEAST forty-five (45) minutes to a vacuum of 29 in. Hg once the lines are cleaned, the new drier installed, the correct amount of Ester has been poured in the compressor body, and the whole system is ready to go. Harbor Freight makes a cheapy high-vac pump for $15. You can hook this up to a spare R134 can adapter hose ($5) to make the correct fitting for attaching to the car. And yes, you MUST evacuate the lines. This is the most common mistake people have in retrofitting. If you don't get that vacuum down in the system, there is no way the air is going to blow very cold.
17) Charge the A/C system with R134a. Generally, about 5% (by weight) less than the R-12 charge amount is required. That means just a little over 2 pounds for a DMC.
18) Check the A/C system operating parameters. The system should function correctly within acceptable limits of temperatures and pressures -- this will ensure that the correct amount of R134a has been charged. I purchased at a small R134 gauge at Pep Boys for less than $5. It looks much like a tire pressure gauge. It tells you the system pressure in terms of low/good/high/danger. Likewise, they have a small & cheap "meat" thermometer that will tell you vent temps inside the car.
19) Replace all R-12 compressor labels with retrofit labels per SAE J1660 in order to provide information on the R134a retrofit which has been performed. In other words, you really should put a sticker on there that says it's R134a. The recommended places are on the compressor and the accumulator.
- In extreme circumstances when expected cooling performance cannot be achieved and high discharge pressures are experienced, it may be necessary to add more condensing capacity to the A/C system. I believe PJ Grady sells a larger capacity condenser for our cars.