- The priority valve has four ports, two of which ( feeds to front rear height correctors ) are closed by a slide-valve when there is no pressure in the system. - When pressure is building up in the system priority is given to feeding the front brakes, - When the pressure is sufficient ( 110 - 130 bars ) to overcome the pressure of the slide-valve spring, it is moved across and opens the supply to the front and rear height correctors.