Western Star air system problems

Okay I have a 2014 Western Star with a cat C13 acert engine the truck has been sitting inside our shop for approximately six months now. Past 3 weeks I've been able to get to it I put on the bushings underneath. The problem I have is when I fired the truck back out for the first time in 6 months the system will not air up I keep getting a reserve air like come on red circle with an r in the middle with three arrows pointing to it. Can anybody please give me some advice what the problem might be where to look. I've already replace the dryer extremely clogged. Thought that was going to do it but to no avail. Any help I can get I greatly appreciated thank you