Add the nitrogen just like adding the refrigerant. The same hook up works. Be 100% sure to vacuum that line from the nitrogen bottle like you would with refrigerant. I don't go more than 150 PSI pressure on the nitrogen since it is both the high and low sides and because 150 PSI is plenty of pressure to look for leaks.

A system under "perfect" vacuum is still just at one atmosphere pressure. A small leak can be hidden at one atmosphere or -14.7 psi. 150 PSI will likely make it leak, and if you are lucky with a soapy water mix you can find it and fix it.

I may well be in just total left field and wasting my time, but hey, it is my time and I prefer to be way beyond "pretty sure" there is no moisture in there.

The system can never be too good of a vacuum, or too dry of moisture. Less is more in this case.

Just my non professional untrained opinion and I give a double your money back guarantee on what I charged you for my advice.

Thanks Glen, I like the idea of putting a good amount of pressure in system and then leak test. I have seen couple videos and they only get pressure to 3-4 psi. I guess if you KNOW there are no leaks that's fine.

Like you Buck, I've taken a multi-year expedition to get cold air coming out of my vents (as fellows here can atest to.) I went so far as to set up a digital system that displayed, among other temps, a core and peripheral (by the dischage port) temp inside the evaporator. From this set up, 2 discoveries were made. First is that the core operates 10 degrees cooler than the peripheral location noted. So it's possible to "trick" a system in operating cooler---at the vent---by having the thermostat probe placed in the warmer area of the evaporator. I say this not knowing where your thermostat probe is placed.

Secondly, and not to add a can of worms to the process of sorting your AC out, I've concluded from "experiments" using a digital thermostat there being a high probability of stock thermostats being guilty of LIMITING the cool an AC system is "allowed" to deliver. I was tempted to swap-out my digital thermostat (which could be programed to allow the evaporator to cool to any degree and know what that degree was) with a stock factory thermostat. With my AC finally delivering cold air after a 10-year expedition, proving or disproving this theory did not have enough appeal to get on with it. Bottom line... my thermostat-guilt theory stands unproven.

Perhaps Charlie would comment on the workings of stock thermos. In particular, what temp are those set to by the factory? And what does the screw in back do?

BTW, last pressure switch I installed... that slowly leaked out where the wires went into it. My AC saga... also continues.

I do have Charlie’s thermostat and fan speed controller in my car. I love his thermostat in that, if rotated all the way to the left it has an off position for the compressor. That way at an Autocross, I can run the evaporator blower to blow air on me, but no compressor running.

The temp switch comes with an LED light that tells you if the compressor is running or not. If you see it cycle on and off in normal use, all is well, and it will not freeze up.

Buck, I did not watch the video you posted, but am assuming you realize that you do not run the a/c compressor when filling the system with nitrogen?

I recently nitrogen leak checked & purged a mini-split system I installed - filled the system to 200 psi and watched pressure gauge for an hour or so while checking all fittings multiple times with soapy water applied with a small paint brush.

After I did the initial pressure hold leak test, I then "swept" the system three more times (per instructions) . . . to sweep you fill system with nitrogen just to the point of breaking the vacuum (do not need to pressurize the system each time), and then pump it down with vac pump, and repeat.

After the final sweep, I let the vac pump run for about 3 hours . . . around 5 hours of vacuuming in total.

Another “My brother” story.
My brother is an AC guy and swears by using a micron vacuum gauge. IIRC, if he doesn’t get to 10 microns he’s not confident of the tightness of the system.
Like OP’s, he charges w/ nitrogen to about 200 psi and uses liquid leak detect.
If all is well then put in a partial charge and observe the pressure over time.
Use a freon sniffer to look for leaks if there is any pressure drop.
His golden rule is “If you can’t get to X microns vacuum, (pretty sure it’s 10) then the system isn’t tight.”
Hopefully some other AC types can chime is so we can all learn.

UPDATE...the Nitrogen purge/flush was the key! I should have listened to Glen, Rawknee and others and done that years ago! I knew things were working when I was able to put 2 cans in and was not getting crazy high pressures on both the low and high side in 3-4 minutes like I was before. Got vent temps around 40 degrees, although ambient was in the 70s so not spectacular. I am short about 3/4 of a can which I may add later to see how low it'll get to. I almost wish it was 98 again here in Dallas just to see what it's like to drive around an not be miserable! Of course as the saying goes about weather in Dallas..."It's either hot, or fixin to be hot".

Good news.

My AC sitting in the garage is never as cold as real world conditions of driving the car.

With my AC at max cold on the temp control, and max fan with the hurricane blower feels COLD driving along.

That's awesome to hear, Buck! I bet it felt great when you were able to tell that actual cold air started moving out of the vents! :)