DD16 westernstar. Air issues! I changed the governor. It’s a tank truck but while he’s idling at 1000rpm he’s lose air overtime in about 15mins. Starting at 120psi then goes to 70 with low air warning going off. I did tell him to run higher rpm while idle but still did the same thing. It keeps doing this I’m stumped. I’m leaning to air compressor. But others saying air dryer but far as I know the purge valve in it isn’t sticking open to blow all that air out. Any ideas?