2004 Escalade - Dual evaps; cold rear only

[SingleMalt]

Vehicle: 2004 Escalade
Ambient: 70deg F
Refrigerant: R134

Hi guys...just registered, but I've been lurking and learning for a while. I've been pretty good at diagnosing and repairing issues with my other vehicles, thanks to the info on this forum, but this 2004 Escalade has me quite puzzled right now. This car has two evaporators, one in front, and one in the rear. The only time I get cold air is when both the front and rear are turned on, and then I only get cold air from the rear vents. I've tested the three possible scenarios and taken some readings:

Scenario 1: Front on/rear on, cold from rear vents only
Stable pressures: 35/200 (yes, no compressor cycling for the hour I sat there looking at it, weird)
Minor flutter on the gauge needles

Scenario 2: Front off/rear on, no cold from rear vents
Stable pressures: 70/75, compressor does not turn on (note the high low side @ 70, weird)
No needle flutter

Scenario 3: Front on/rear off, no cold from front vents
10 second compressor cycles. On at 50/120, off at 25/150.

Scenario 3 seems to indicate low refrigerant due to the rapid cycling, but on the other hand, those pressures seem in the right range for the ambient. Am I missing something obvious here? One other bit of information that may or may not be relevant: there is a continuous hissing sound from the area of the accumulator. Not sure if this sound has always been there or not. It is NOT a leak however, as I've injected dye and UVed the area, with a negative result.

Many thanks.

[bohica2xo]

Rear system only runs if the front is running.

Rear system has a TXV. Front is Orifice Tube. Recover the system, & check the OT.

.

[SingleMalt]

Well, gee, that makes sense when you put it that way I was indeed missing the fact that the rear system does not operate when the front is off. I had kind of suspected the orifice tube - thanks for the confirmation. Will pull and inspect.

[SingleMalt]

I did the work this weekend. Pulled the old OT, slipped in a new one. Replaced the schrader valve core on the low side and ball valve on the high side while I was there. Nice and cold front and back now.

Unfortunately, I get a freon smell from the front air stream, that I do not get from the rear air stream. Probably a leaking evap in the front...the original is still in there, so not unusual for a vehicle of this age. But, that is a project for next spring.

Thanks for the help.