to do a retro on a running system, i drain the compressor of oil, replace the dryer and expansion valve, while the system is apart i blow as much oil from the system with shop air as i can, (now that i have a flush gun i flush the evap and condensor with ac flush solvent, $125 from napa for the gun/tank and a gallon of flush, i use a 1/2 pint at a time it's $50 a gallon) replace the fittings with quality steel fittings with their own valves not those shitty aluiminum ones that use the old valve, the good ones you remove the core from the old fitting before you screw on the 134 fitting on, refill the compressor with about 6 ounces of ester ac oil and vaccum the system down for about a half hour and charge with 3.4 pounds of r134
all america wants is cold beer warm cat and a place to take a poop with a door on it