Eldex pumps are designed for constant flow rates, not constant pressure.  Pressure is a byproduct of the flow, i.e., higher flow with the same outlet restriction results in higher pressure.
Pressure is fluid delivery resistance (on the pump's output side). The pump does not generate any pressure but can move fluid despite the resistance to fluid movement generated by something else (an HPLC column, tubing, etc.).
Read our troubleshooting guide blog post for additional information on piston pump flow rates.