Designed for use on dual-air system vehicles, the ratio valve (Fig. 24) is installed in the front axle delivery line. During normal brake applications, this valve automatically reduces application pressure to the front axle brakes. As brake application pressure increases, the percentage of reduction is decreased until about 60 psi (413 kPa) (depending on valve design) when full pilot pressure is delivered. The valve is available with several different “hold-off” pressures, which prevent the front brakes from operating until this “hold-off” pressure is exceeded.
Note: Older trucks may be equipped with a front wheel limiting valve controlled by a switch on the dash. When activated, this valve will reduce application pressure on the steering axle brakes by 50%.