Primary air tank won't build air

[in2trux]

Sounds like the compressor itself, sticky unloader. You could try wd40 in the intake of the compressor but you have to open the intake line at the compressor head.
I would also check the purge valve on the drier while it's building air. I've had them stick open and not be able to build air. Leveler valves for the airbags can also bleed off allot of air but they should be on the secondary.