Relay valve. Might be full of trash, no air to it. More often than not it's gotten crap in it and the diaphragm is stuck. Clean it off and determine which is main supply (might be marked reservoir), usually a large line in, and see if it's got air. Next find supply line and see if you have air when you step on brake pedal. If those are good I'd say it's fubar.
The smaller lines out will be release air for springs and apply air from pedal. They're not very expensive. If something is acting up I usually just change it.