98 Suburban AC only cools when rear fan switch on

I have a 98 suburbanand recently noticed it only blows really cold air in front when the rear temp control fan is turned on.

Front AC switch on, front fan switch on (speed doesn't matter), rear fan switch off.Used a temp gauge in the centervent and the air does blow colder than outside ambient temp but not much~70 -75 degrees. The compressor is on. As soon as the rear fan switch is turned on (low), the air in the front vents starts to drop until it reaches around 48 degrees where it stabilizes. The rear also blows cold.

For the above scenarios, the front re-circulate switch and front fan speeds do not change the behavior. (e.g. the front only blows cold when the rear fan switch is on).

Measurethe low and hi pressure cooling system sides last night and results below. The AC compressor was replaced several years ago and have not had any issues sincethen until we noticed this.

Any idea what may be causing this?

switch positions Low Press Reading High Press Reading Notes[/align]front on/rear off25 175 compressor on[/align] 45 125 compressor kicks off when rev'd for a while when low press drops to about 20[/align][/align]front on/rear on 30 200 idle[/align] 45 240 rev'd - compressor doesNOT kick off[/align][/align]front on warm/rear on 45 225[/align][/align]

 

Sounds like it may be the control switch unit.