99 Volvo cummins engine automatic

Reading that made my head hurt.

Try rewriting this post using punctuation, as your question is one big Run On Sentence that's hard to understand.

 

That's confusing but hopefully not so complicated as some old trucks with a 721 system and five air tanks.
The basic requirement is three air tanks. Wet tank, primary and secondary. Some times the wet tank is combined in the same tank with one of the others. There is a divider in the middle and a check valve.
The air governor has a signal line to the primary or secondary tank. The compressor will not engage intel the pressure from the signal tank is about 90 PSI. There are check valves so air cannot back flow from any tank to another. I suspect the check valve for that tank is sticking, air pressure is not equalizing, and the compressor does not come on because the signal line is connected to the other tank. Find and replace that valve.
That's what I thank but there are all kinds of different variations these days. Governors on air tanks and such. Good Luck and let us know what you find.

 

No more likely a check valve on an air tank or something similar to that. Your problem is lower air pressure in one tank than the other and they should be equal or close to it.
KISS principle: Keep it simple Sam. Used to be stupid but that's not politically correct.
Air comes in that tank from one hose that is some how connected to the other tanks. If they do not have the same pressure the gauge is wrong or there is a restriction suppling the tank.
Everything on a truck can make it seem overwhelming but keep it simple. Three hundred feet of garden hose coiled up without much water flow can be caused by a single kink. Might take a while to work through it but often pretty simple when you find it.

 

could be pressure regulator valve though that adjusts the pressure that the tank supplys and adjusts the gauge pressures or try the little signal line that goes back to the compressor though i think its further down the line somewhere