Underdog that is incorrect there is an unloader valve on the compressor that controls the compressors cycles. When an airbrake system dumps that is an air dryer expelling moister from the system. On a properly operating system the compressor will stop at operating pressure between 90 to 125 psi. It is true that the external pulley or gear will remain turning but the internals will stop most work an internal clutch through a pushrod that rides in a bore at the back of the crankshaft controlled by the unloader valve on its cylinder head. If you listen to the compressor as it builds air you can hear it stop when it reaches its set pressure and restart when that pressure drops to low.
