What problem would cause a rotary pump to have an excessive vacuum reading?

An excessive vacuum reading in a rotary pump can often indicate issues related to the ability of the pump to maintain its prime and effectively draw in fluids. When the foot valve poppet spring is too heavy, it can prevent the valve from opening fully or at all, restricting fluid flow into the pump. This results in an inadequate supply of liquid, which leads to a vacuum being created without sufficient fluid to offset it. The vacuum gauge will reflect this negative pressure as the pump struggles to draw in the liquid, which is the primary reason for the excessive vacuum reading.

In contrast, the other options generally relate to other mechanical issues or system configurations that may not directly lead to excessive vacuum readings in the same manner. A broken suction line would typically manifest in a loss of prime or air leaks rather than just excessive vacuum, a stuck bypass valve could cause overpressure situations, and a needle valve issue generally pertains to incorrect fluid flow regulation rather than vacuum levels. Thus, the heavy foot valve poppet spring offers a direct mechanism for creating excessive vacuum by hindering fluid intake.