To maintain the stable partial vacuum needed for milking, the vacuum pump must continually remove air at the same rate air enters the system.
Air entering through the pulsators, claw, system leaks and milk meters, if present, is quite steady.
However, during milking, air entering the system will vary considerably. Attachment and detachment of units, squawks and unit fall-off all admit air sometimes in large quantities. Air flow needed for washing has generally been thought to be greater than that needed for milking. This is changing. Traditionally, vacuum pumps have been sized for the worst case with a considerable safety factor.