Thank you again, you've been super helpful. I am now a bit more confused about the leak. I'm not so sure that it is the evaporator anymore.

I vacuumed the system for an hour last night and left the vacuum in the system for about an hour more. A considerable amount of the vacuum was gone. Just in case I had excess solvent I ran the vacuum overnight and confirmed I was still leaking when I tested the vacuum again. I read about using nitrogen or running soap water over the lines but using dye sounded easier. The leak was confirmed again though after adding r134a today. I could see the high side psi dropping.

I want to start by saying I just ran another quick test. After 6 hours the system is holding about 70 psi in both the low and high. With the van on the ac clutch cycles and the high side is about 110 psi and the low 25 psi. Why this last bit is not leaking is confusing. I'll test again in the morning.

• Exactly how much refrigerant I added today is difficult to say due to my connectors leaking. I went through 2 12 oz dye cans and some more in a 3rd can. I also tried to use a larger walmart can but I'm not so sure that can released any r134a. The compressor stayed on and somewhat cold air vented to the cab. Temperature today was about 70 degrees. I briefly tried charging through the high side knowing I shouldn't but I couldn't get the clutch started and psi was low.
• I picked up the following vacuum and hose kit on Amazon but the yellow hose is for I believe R12. The adapters I purchased from Lowes leaked. I tried teflon tape, a sealant, and an o-ring but they failed me. Once I switched to a cheap hose I had from an old Walmart kit I was able to charge the system. The gauge kit does not appear to leak and I never tightend any more than by hand.
  Amazon
  Amazon
• And correct, blue hose to low side, red hose to hide side, yellow hose to cans/vacuum. All but a few seconds of the r134a was fed through the low side.
• The evaporator box was only held on by about 8 small bolts. It then pulls apart and I could slide the casing off. The evaporator itself appears to slide right out after I detach the AC lines. A shine of my UV light revealed what looked like dye on the evaporator and plastic encasing. After looking with the sun down I don't believe it is dye. (Though it really could use a cleaning.. full of lint and debris)
• Great ideas about the process of elimination. I'll give that a shot. With the rear AC there are so many places it could be leaking.
• Your description of the gauges and how the psi changes matches what I saw. My system was stock r134a and per spec it states 64 oz. That's what I was going to go off of. Refrigerant and oil capacity charts - E to H || TechChoice Parts
• The accumulator / dryer contains the low side suction schrader valve so it was replaced with the new unit.

My leaky connector shot dye everywhere which makes it a little more difficult to track. (I'm ordering a better connector with a built in o-ring) However, I don't see anywhere that looks like an obvious leak. Any idea why the system holds 70 psi now but clearly lost pressure earlier? It lasted about 15 minutes before the clutch started cycling again at a lower pressure.

EDIT: Today, (following morning), the psi has held steady and not leaking any more at this point.

 

Geez this got longer then I expected!
I will write another post answering some of your other questions...

“Why this last bit is not leaking is confusing”.

The 70 psi equal on both sides with the AC off is what is called static pressure.
Your outside temperature should have been around 69-70f at that time when you checked the pressure and the engine was cold?
I am not a professional here but this is my understanding about static pressure. Since you are using R134a I will use it as the example.
R134a freezes at -153.9F and boils at -15.34F and turns into gas.
When adding R134a into a enclosed enclosed AC system that is off, as you add the freon from your container, the pressure goes up inside the system till it reaches a point to where the R134a can no longer evaporate or boil into a vapor. The pressure at which this happens is dependant on the ambient temperature. So when you first start filling the system from a can, turned upright so no liquid is going into the AC system, only gas, because the pressure is lowered in the can, the liquid freon in the bottom of the can is actually boiling and turning from a liquid to a gas.
Just like boiling water in a open pot, eventually the liquid water boils off till there is no more water left in the pot, the liquid R134a in the can will boil off till there is no liquid left in the can or in this case, until the pressure equalizes between the can and the enclosed AC system.
Once this happens, that is your static pressure.
The static pressure of R134a with a ambient temperature of 70F is “71.2psi”.
SO if you could close the valves on the gauges and disconnect the hoses without loosing “any” gas from your AC system, you would have 71.2 psi of gas in your system at 70F.
If you have a very small leak, the pressure in the system will steadily drop till there is no gas left in the system at all.
Now remember that we only put gas in the system, no liquid.

Now what if you have liquid freon in the system along with gas?
The static pressure will not start dropping until all the liquid in the system has boiled off.
So depending on how much liquid you have inside the system and the size of the leak, that determines how long it will take for the liquid freon to boil off and the pressure start dropping.

So when you said: “Why this last bit is not leaking is confusing”. What you are seeing is the static pressure and there is still liquid boiling off inside the system. If the temperature remained at 70F, the static pressure would remain at 71.2psi until all the liquid has boiled an turned into a gas.

So what can static pressure really tell you?
It can tell you if you have freon in the system but it can not tell you how much.
IT can not tell you if you have a leak or not “until” the liquid is boiled off.

Below is a chart that shows you what the static pressure of R134a is at different ambient temperatures. Keep in mind that the chart only applies to ambient temperature. If you have had the van running the engine compartment is hotter and so are the compressor and other parts. That will raise the pressures.

 

Really really helpful. That makes perfect sense. I'm going to watch the lines and parts for leaks over the next few days. Thank you.