Ok I have a 06 2500 with the 5.9 Cummins. Has new compressor dryer and lines the hi side won't go past 75 and the low side is at 80 with the valve open and when you close it to check the pressure it's at zero. When I shut the truck off the low side goes up to 120 and the hi side stays the same. Any suggestions?

 

My first suggestion is try a differnent set of gauges or test them on another car to make sure they are accurate. If they are accurate, most likely you do not have enough r134a in there or the compressor, even though it is new, has a worn out high side valve. This is normally caused by not flushing the system when a compressor dies, or running it slightly low on oil.

 

X2 - with the truck running and ac on full blast add freon and compressor will kick on to move air in the system. Once air is cycling fill correct amount