After much of the troubleshooting you describe, I replaced the compressor, orifice tube, stock ac dryer in the engine compartment, and expansion valve to the rear evap.
I do not think that there was any oil in the system. I did not see any anywhere and no dye at that point. After some investigation, I found that the previous compressor was a brand that does not send the compresor charged with oil, and I think whoever replaced it previously though it had oil, so they did not add any.
After replacing the above stuff...
Upon trying to charge the system, it stopped at about 2.5 pounds, when the sticker under the hood called for 6. I did not have a bottle heater and was strapped for time, so i had to wait. I decided to order a replacement evaporator, cahngee it out and tried again.
I found the carrier procedure of adding liquid to the high pressure line while the system was off, followed it, and got the full 6 pounds in. I let teh bottle warm up before starting the engine, so it would likely take gas if I needed to top it off.
In that procedure, there was a chart to adjust the charge if the high pressure side did not reach a certain number, calculated from the ambient temperature, with the engine at 1200 RPM.
I added gas until the high pressure was in the correct range ( ~145 psi at that ambient temp), but this brought the low pressure up to 60 PSI.
Both front and rear units were cooling 70 degree air to about 60 degrees. The air from the condensor felt a little warmed.
BTW, the reason I had not changed the condensor coil, was I could not find the part number available to order.
I did change the sight glass/filter/dryer under the bus after the condensor. The sight glass showed a solid stream of bubbles. ( really foam) As my assistant would try to maintain 1200 rpm, the stream would sometimes be clear liquid, as the rpm fluctuated up and down.
The compressor is brand new and was not mistreated, so I think it is doing all it can, and that it is just not enough.
The only other theory I have is that the condensor might have a few of the parallel branches plugged, so it is all going through one branch, but i really doubt that.
I certainly do not know for sure, but it seems that the compressor just cannot keep up.
I have not tried to run with just the front ac yet, because after doing the testing above, I had to go to work and the schrader valve on the low pressure side decided to leak horribly, so I put my gauges back on it quickly to keep it from leaking out, and while waiting for another chance to work on it, figured that the compressor probably does not have high enough capacity to pump all the gas that both of these evaportors dump to the low side.
That is where it sits right now. Carrier ( now MCC ) cannot tell me what size compressor should feed the evaporator in this vehicle, or even how many BTU's it can handle.
I really am hoping that it just needs a bigger compressor, and someone out there has that answer.
I am prepared to try other things. I never really pretend to have anything figured out until it has been working for a month or two.
Proper high pressure and elevated low pressure, it just seems that either the resistance from high side to low side is too small, or the pump is too small.