Systems and methods quickly regulate hydraulic fluid pressure to meet hydraulic fluid demands in subsea equipment over long umbilicals. A hydraulic power unit includes a reservoir and a pump for pumping hydraulic fluid from the reservoir into a fluid supply line to subsea equipment. A supply-side module located downstream of the hydraulic power unit includes a circulation pump in communication with the supply line for circulating hydraulic fluid in a circuit including the fluid supply line and a fluid return line. A demand-side module is located upstream of the subsea equipment and includes a diverter valve for opening or closing the circuit. Adjustments can be made to the circulation pump and the diverter valve such that hydraulic fluid is circulated through the circuit. The pressure of the supply-side module is greater and does not exceed twice than the pressure of the demand-side module.