Hydraulic valves Thehydraulic valvesare part of thefamilyof the control devicesandregulators.They work onpressure, flowor referralof the fluid throughthecircuit.Thevalvesare also usedforthereturn of the hydraulicfluid in the reservoir.Thevalvescan controlthe needto start orstopa hydraulicactuator.Wedistinguish thevalvesinto two types:continuous-actionvalves(proportional orservo)andvalvesall or nothing.