If you pump liquid in a closed loop, and you attach a reservoir that communicates with the atmosphere, that reservoir becomes sort of a reference point. If it's attached to the pump suction, the entire loop will run at a positive pressure. If it's attached to the pump discharge, the entire loop will run at a negative pressure.
... after everything stabilizes.
... roughly.
... absent other influences.
But such pump work is not normal, right ? (isssues such as cavitation, etc : pump sounded indeed terribly, and I did not dare to start the chiller today for the season). There often were found opulant brown foamy exudes from a degassing valve - I suspect microorganisms in the system - hope it's not legionella.
Chilled water loops should have an expansion tank at the low pressure point in the loop to accommodate the changes in fluid volume with temperature changes. This tank normally has a water pressure regulator near by that will add more water if pressure drops below set point (often 15 psi). It will also have an air vent to vent excess air that might accumulate.
