When the rack moves it turns the pinion, which moves the sleeves, in which the control slide- valves are situated, in the direction which would tend to make the valves return to the cut off position. As long as the driver turns the steering wheel he holds the slide-valves in the open position, but when he ceases to turn, the sleeves return to their cut off position in relation to the slide-valves and the rack stops moving. 4. Residual Pressure :