When the air compressor reaches preset upper pressure, the governor will shut off the air compressor and air is released via the purge valve on the air dryer. That is normal. If it fails to build up pressure, or is taking a long time to recover from brake use, then it might be an issue. How about when the engine is off, how long does it take air pressure to bleed off? Do the air springs hold air overnight? Do you hear air leaking out after stopping the engine?
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI