2008 cat SVP?

Not exactly sure on the engine model but it was one of the last cat engines. Anyway I’m having issues with the air building. Primary builds at a okay time but still longer than 3 mins it’s at like 60 at 8mins then finally secondary starts moving and I believe truck gets full air around 15mins revving at 1200!
I changed the unloaders. Fairly newcompressor by looks of it! And an air dryer filter. But if it takes forever to build insecondary what could be the issues?not air compressor since it dies build primary ok just not secondary