Which USC brand/model do you use? What kind of fluid in it? I looked into them once, and reviews were mixed, but with enough bad ones over enough models, that I held off.
A hack like me uses a garden hose with a brass nozzle. It seems to do a very good job of getting them clean. For me compressed air isn't worth the mess of dust everywhere for what little it cleans. After the hose, I inspect every surface with either papaer towel, finger, or cotton swab for any fine dust or polishing compound residue. I'll use soap before hosing and again during a wet inspection/rinse if it is a cylinder that has been run or had oil in it.