an electrically driven solenoid coupled to a spring - operated valve , regulates pressure in an accumulator by opening when a predefined threshold pressure in a pressure accumulator is exceeded . the solenoid provides an assistive force to a spring - closed valve , reducing the amount of pressure required to open the valve responsive to the amount of current provided to the solenoid . the threshold pressure at which the valve opens is thus determined by the amount of current provided to the solenoid . increasing the current decreases the threshold pressure ; decreasing the current increases the threshold pressure .