a pump for use with a dispensing unit of a liquid hot melt adhesive system , dispensing units including the pump , and a method of operating a pump for pumping a liquid like liquid hot melt adhesive . the pump has a piston movable within a pumping chamber for transferring liquid from an inlet to an outlet . a valve body , which is resiliently biased relative to the piston , is moved out of contact with the inlet when the piston moves in a first direction and into contact with the inlet when the piston moves in a second direction . when the piston is moved in the first direction , the resilient biasing actively lifts the valve body out of contact with the inlet for overcoming any attractive adhesive force acting between the valve body and inlet .