The servo has a control valve (19) operated by a solenoid (20) to connect a pressure line to a chamber in the servo. The solenoid operates a plunger (28,29) to lift the valve element (44) off the valve seat. The improved drive for the plunger has an initial. lower current for the solenoid, sufficient bring the plunger into contact with the valve element and a second, larger current to open the valve. The first current level is also sufficient to counterbalance most of the spring pressure on the valve element and is applied long enough to enable the plunger to come to rest against the valve element. The initial current can be a constant current or can rise steadily from an initial current to the final operating current of the solenoid.
