Foot brake valve leaking

[Redland1]

2003 International 9900I. The foot brake valve has been leaking for about two weeks and when the truck is shut off it will completely drain the tanks. Also there is a squeaking sound when pressing the brakes most of the time. Today after work I turned off the truck and when I turned it back on the air pressure wouldn’t go past 30 psi unless I pressed the brake and held the accelerator. Any ideas what I should be looking for ?