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The system is designed to be filled from a vacuum. I've never tried pumping goo juice from vatozone into system that's at atmospheric pressure but I'd imagine that you'd struggle to get the proper amount of freon in there and your pressures will be way off, resulting in shitty performance and potential future damage.
A cheap vacuum pump and gauge set is like $150. It's well worth the money. I always replace shrader valves, every time. They always leak. Complete kit for that truck from 4 seasons is probably less than $200. For $350 you'd have a new compressor, accumulator, hoses. You'll know how much oils in it, and it will blow cold for another decade or two.
Or keep pumping $25 cans of crap in there until it explodes.
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You don't know much about refrigeration if you think a system cannot be easily charged when under pressure. I will admit that measuring charge into an evacuated system is the best way, but if the system still has a partial charge, systems can be charged from a partial remaining charge if you can establish some conditions that are relevant. In other words operating condtions thatr would be realistic, and factory pressure readings re: wet/ dry bulb and so on. Frankly, the singular most important thing you can do is to insure that the compressor sees adequate superheat so that it does not slug. Probably second is to insure that it does not see excessive head from overcharge, and of course that means subcooling.
I see NO need to evacuate a system that for certain is still under pressure, and one that you feel has not been subjected to a vacuum. Most any system with a low pressure cutout and that has not seen winter low temps will meet that, with a bit of common sense.
The biggest issue with automotive systems is the wide operating conditions they face.
Also you can easily (usually) replace Schrader valves with the proper tool. The first one I bought, for common 1/4" flare service taps, was about 1982, and I still have it. Since then, I've acquired several others.
You can easily remove most of a charge without a recovery system by simply using a recovery tank which you've made sure is "clean" and evacuated, and then left in the freezer overnight. All you need do is get the system down into the vapor only pressure range, and low enough that you can deal with it, say 15--20PSI. Then you can change Schraders WITHOUT the special tool, and simply use your finger to hold pressure, just as iff you were doing so on a tire.