Splitter not working, air pressure or trans problem?

I have a 93 Kenworth W9 with the 15spd trans(10 regular speeds and 5 spd deep reduction). The truck has been setting awhile and I went to take it out yesterday and the splitter doesn't work, won't lemme into high range. Almost sounds like there's not enough air but the gauges on the dash show full pressure. The deep reduction switch works intermittently but when it does work you can sit there and switch it all day long back and forth(and hear it engaging at the trans) until you go to hit the splitter for high range then the deep reduction switch won't work for another few minutes. When you hit the splitter for high range there is a faint hiss of air at the switch but not like normal and there is a real faint clunk at the trans but also not like normal. It's almost like there's not enough air constantly making it to the switches on the shifter.

Took the cover off the shifter and there's 4 air lines: RED; BLUE(deep reduction); BLACK; and GREEN. Anybody got a diagram that might help? Any idea as to whether this is an air pressure issue or a mechanical issue at the trans?