If a water pump is to create a partial vacuum in the pump housing, three things must happen:

  1. The pump must be primed. The water in the housing is essential to lubricate the mechanical seal so that it won't wear and leak.
  2. The suction hose, hose seals and all O-rings must be in good condition so air can't be drawn in, losing the vacuum.
  3. The impeller-to-volute clearance must be within specification to achieve the proper vacuum.