Ok What I would do is pull the out line off the compressor (Im assuming its a belt driven,sorry I have no knowledge of 345's,but aircompressors are aircompressors) and check if the compressor is indeed pumping. Sometimes the outlet will get carboned up and plug up. If not then it maybe the governor. Goveners are not hard to change.For this part you may have to go to a truck dealer, like KW,Pete, Freightliner or GMC. If it none of the above then there is a valve on the axle that can go bad.
In any case if you are not able to trouble shoot yourself, its best to take it too one of the above mentioned places. They are well versed in air-ride systems. Now if it has an electric compressor I cant help ya.
Roger & MaryLou
F350 CREWCAB SW LONG BED
7.3 liter Power Stroke Diesel
1977 27ft OVERLANDER
AIR # 22336 TAC- OH-7
May your roads be straight and smooth and may you always have a tailwind!