These systems were leakers from the factory. Chances are very good that the system is already empty and you won’t need to “recover” refrigerant that leaked out long ago.

I assume you’re converting to R134A refrigerant when you replace the compressor? If so, also replace the a/c hose assembly with new a R134a hose assembly. R134a will leak through the original hoses. Flush the condenser and evaporator with a solvent designed for a/c systems, then blow through each one with DRY compressed air or ideally nitrogen. Replace the accumulator. Replace your original fan clutch with a quality replacement part. Replace the orifice tube with a red Ford orifice tube. The factory low pressure switch (that thing on the accumulator with the wires attached to it) is adjustable. There’s a screw between the two switch terminals. You will probably have to adjust it so that the compressor shuts off at 27 PSI, but you can’t do that until the system is charged and running…

You’ll need to buy and install two retrofit fittings in order to connect testing/charging equipment to the system. DO NOT buy those shitty aluminum adapters that “convert” your factory threaded fittings to accept R134a quick connects. Buy the good steel fittings and remove the valve cores from the factory fittings prior to installing the retrofit fittings.

Your new compressor may be shipped filled with oil or it may be shipped dry. If it’s dry, make sure to fill it with the oil specified by the compressor manufacturer. There’s usually a tag or label on the compressor that calls out what oil to use and the system’s oil capacity. If not, email the compressor manufacturer for that info. If you need to add oil, don’t add it all into the compressor. For example, if you need to add 4oz. of oil, add 2oz. to the compressor, 1oz to the accumulator and 1 oz to the condenser. Oil circulates with the refrigerant. You don’t want to dump it all into one component and create an oil slug. That could ruin your new compressor.

Once you’ve replaced all the parts, turn the compressor clutch hub about 40 revolutions by hand. This will expel any excess oil from the compressor.

Remove the valve cores from the service ports in the low and high side. IIRC, the high side service port is located in the liquid line on your truck, and the low side service port is on the accumulator. After the cores have been removed, install the retrofit fittings onto the ports. Connect your gauges and vacuum pump. Let it pull a vacuum for an hour, then close the valves on your gauge manifold and note the low side reading. It should be 30” of vacuum. Walk away for an hour, then recheck the low side gauge. It should still read 30” of vacuum. If the gauge has moved, you’ve got a leak. If not, prepare to charge the system.

There should be a decal under your hood that shows your system’s R12 capacity. If not, Google is your friend. If memory serves, it’s about 3lbs. Multiply your system capacity by 0.6. That’s going to be slightly less than the amount of R134a you’ll need. R134a is sold in 12oz. cans. Buy one can MORE than your calculation suggests you’ll need. Connect the can to your manifold set and open the valve on the can, then open the low side valve on the gauge manifold. You should hear refrigerant flowing into the system and see the low side reading rise. The high side will also rise, but much more slowly.

Place a jumper wire across the terminals of the low pressure switch connector. Start the engine and turn the a/c to MAX, with the fan on 1 speed below HIGH. You should hear the clutch engage the second you turn the key to the on position Place a duct thermometer in the center dash outlet. The system will draw in the refrigerant. Watch the low side gauge as the system is charging. It should read around 50-70PSI while it’s charging, then drop to anywhere from 0-20PSI when the can is empty. After you’ve put two full cans in, remove the jumper wire and plug the connector into the low pressure switch. Your clutch should now cycle on and off.

At this point, prop the throttle open to achieve about 1800 engine RPM. SLOWLY start adding the third can, adding a little bit of refrigerant at a time. To accomplish this, turn the low side valve on your gauge manifold on for a second or two, then turn it off and let the reading stabilize. Continue slowly adding refrigerant until the low side gauge reads about 32-35 PSI. “Normal” pressures change with temperature, so if it’s really hot outside, expect to see 40-45 PSI on the low side and maybe 240 or so on the high side. If it’s 70F outside , expect to see 32-35PSI on the low side and less than 200PSI on the high side. Check your thermometer. The center outlet temp will ideally be about 37F, but that’s often an optimistic goal with a retrofit. You’re golden if your duct temp is 37-45F. Turn the engine off.

Note how much refrigerant you’ve used. Install the retrofit label. Disconnect your manifold set. Install the caps on the service ports. Cruise in comfort!