A/C Leak(?)

Let's start at the beginning because I think you are basing this on some assumptions. At least that is what it seems with the info that you provided.

So, let's start with what the actual pressure is when the A/C is on and off.

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

Once we know what the pressure is we can take the next step.

However, if the pressure is 0 then the system is empty as you stated. So that means we may not have had an issue when you pulled it into a vacuum but then the leak developed later. So, we need to redo the vacuum test to try and find the leak.

Here is a guide that will help with this:

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

However, the leak may not be bad enough to leak right away so we need to pull it into a vacuum and the system should hold that vacuum for at least an hour.

Here is a guide that will help with finding the leak but if you added dye the last time you charged the system then we can jump right to that step in looking for the dye:

https://youtu.be/yrhWKlQEPWw

Lastly, if you have a leak then you most likely do not need to add oil back into the system. The only oil you will need to add is that amount of oil that comes out when you vacuum the system.

I vacuumed down my system earlier today and charged it with 28 oz. of refrigerant as per the specs. My vent temperature went from 93.3 to 57.2 F. Static pressure ended up at 115 psi., which reflected a temperature of 92 F on the manifold gauges. After the recharge, my pressures were 34 on the low side and 255 on the high side. I used a refrigerant "sniffer", which I recently purchased, to check for leaks. I found none within the engine bay. If the system loses pressure again, what would you suggest?

If you find no leaks in the engine bay, you need to access the evaporator by removing the blower motor and use your sniffer in there.

That is great that you have one of those. It will make this much easier.

However, don't neglect checking the engine bay again. Clearly a leak is a leak so if it is not leaking right now then you would think it is not going to leak but there are exceptions to every rule so just cover the engine bay each time you check it just to be sure.

Okay. The system vacuumed down and charged to capacity four days ago. I borrowed another vehicle until today, which was the first time I could check my A/C. Not even enough in the system to trigger the compressor clutch. I guess I'll need to recharge and look for leaks at the evaporator, which I have not yet done. I really expect it to be the evaporator at this point, though.

I don't disagree. If you found no leaks prior, then that would make sense.

However, you may not need to charge it again if dye was in the system.

You can also just access it and look for oil residue because some of that would have come out. However, Freon with that leak detector would be easier.

No dye. I thought the leak detector would be easier than dye. Since it doesn't leak under vacuum, I'm thinking I should maybe have the system pressure tested for leaks.

Understood. You are correct that the detector is easier, but it requires the use of Freon which can get expensive if it is a large leak.

In theory a vacuum and pressure test should show a leak, but I understand the confusion. If it is empty, then it should not hold a vacuum due to a leak.