So on the pressure vs vacuum, i figured youd want pressure not vacuum because the system operates with pressure so pulling vacuum would not represent the conditions of the system when its filled

And on the moisture thing, other than replacing the dryer, isnt the fix to pull vacuum for many hours?

 

To your first point, you're mostly correct. And what I mean by that is, yes. The system does operate under pressure, but it doesn't "care", or "know", if it's under vacuum, or has pressure (I'm addressing your "...represent(ing) the conditions of the system when its filled"). You don't need to "represent the conditions" at all, and as long as the system is under pressure (100 PSIG or so), you can find a leak with either the refrigerant charge (via a sniffer), or soapy bubbles (nitrogen).

With regards to pulling a system into a vacuum, the amount of time doesn't matter as long as you're able to monitor the micron level (amount of vacuum), and make sure it's stable. Let me see if I can explain it this way: If, for example, you're located in Salt Lake City (very little humidity), and the system is only open for a short amount of time, very little moisture will gain access to the system, and the amount of time needed to pull a "deep vac" will be much less than a similar job being performed in, say, New Orleans, or Houston (much higher humidity). However, if that same system in Salt Lake City is left open to the atmosphere for a long period of time (an hour, a day, a week, etc), moisture will get into the system, and then a longer vacuum will be needed. How long? Who knows!

In other words, in order to do the job properly, it's not the amount of time that a system is put under vacuum, but the amount of moisture that's in the system that needs to be monitored, and thus the need for a micron gauge. Heck, I've worked on cars that only needed 45 minutes of vacuum, while on others I let it run overnight, and then had to change the oil in my vacuum pump so I could run it some more the next day. Everything is relative....

Now I know a lot of people here say, "I put it on a vacuum for (X) hours, and it's working just fine." Good for them, and I hope it runs that way forever!

But no one reports back here and admits they (unknowingly) left moisture in the system and scrambled a perfectly good compressor. Or worse.

 

ah okay good to know. we have a very humid summer up here. fortunately my f150 ac is still working, but its leaking. I have added refrigerant twice by cheating, putting gauges on and purging the lines t othe gauge, then putting in like half a can and sending it. lol

I know its not proper, but its unbearable in the summer when its wet and hot and being that it worked at all was enough for me to dothat lol.
My silverado works too, but the belt is slipping due to age and bad tensioner. gm wants almost 200 for a tensioner for a single belt. im gonna use a stretch belt to make that cheaper on me.

I am also gonna do the mod to turn off the heater core when the f150 ac is on, i also need to fix the check valve for the aC because it will periodically run out of vacuum at sustained throttle and i need to reset my switch when i let off to get it back

 

That's quite popular, and an entirely reasonable thing to do (EPA restrictions notwithstanding), as long as the system has never gone flat, or been pulled into a negative (vacuum) by the compressor (jumped-out low pressure switch, anyone?), thus allowing air (and moisture) to get in there. But if it has (system has gone flat, or has been pulled into a vacuum), be aware that moisture reacts with certain refrigerants, causing it to breakdown and produce an acid...thus damaging the components in your truck.

 

Yes its been full since new beside my topping off, since i had no way of knowing how much weight in refrigerant was inside i only added until my vent temps were back to what they used to be , i suspect ill do the same thing here in a month or so