2002 yukon xl: low pressure ac readings?

Hey guy - sorry if this comes across as a dumb question but I need some help.

I have a 2002 yukon xl 2500 8.1l that had the ac serviced about 3 months ago because there was no cold air coming out of either the front or the rear ac. They shop said the schrader valaves were leaking so they replaced them and recharged. Everything was working great.

Then things started to act up. The front ac would not blow hot air unless the rear ac was on and now both are back to warm air. I decided to get a can of 134a and see if that would help. I put some in and things went back to somewhat cold air coming out of the front but only if the rear was on.

I decided to check the low pressure again and just conne ted the guage only. If I only have the front ac on the pressure with the compressor running is around 25. If i then turn on the rear ac as well, the pressure goes to 45. Does this sound right? When I am adding 134a i assume i should have both front and rear running.

Thanks for any help you can provide.

Yes, front and rear running. 45 should be in the acceptable range depending on ambient temperature, but you really need to get a reading on the high side as well to get a true picture. An infra-red temp gauge helps check the temps on the lines going in and out of the evaporator.

Thanks for the reply. I'll find a set of gauges so I can check the high side as well. I guess it makes sense that the low pressure would be higher with both the front and rear on versus the front only.

Any idea on the front not cooling unless the rear is on?

Yes- overly complicated wiring, tons of switches and sensors.