If a water pump is to create a partial vacuum in the pump housing, three things must happen:
- 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.
- The suction hose, hose seals and all O-rings must be in good condition so air can't be drawn in, losing the vacuum.
- The impeller-to-volute clearance must be within specification to achieve the proper vacuum.