Combination Valve
The brake hydraulic system in this vehicle is equipped with a combination valve. See Figure . The combination valve is used for controlling and monitoring hydraulic portion of brake system. The combination valve serves 3 functions in brake's hydraulic system. It directs (diverts) brake fluid from master cylinder to front and rear brakes.
It controls hydraulic pressure to rear brakes through proportioning function of valve and monitors for a pressure loss in system through pressure differential valve and warning switch. This is accomplished by combining a pressure differential valve, a warning light switch with a single proportioning valve. See Fig 2 . The combination valve is attached to frame rail below master cylinder. See Figure .
The proportioning valve balances front to rear braking by controlling at a given ratio, increase in rear brake system hydraulic pressure above a preset level. The proportioning valve section operates by transmitting full input pressure to rear brakes up to a certain point. This is called split point. Beyond this point it reduces amount of pressure increase to rear brakes according to a certain ratio.
On light pedal applications equal brake pressure will be transmitted to front and rear brakes. On heavier pedal applications pressure transmitted to rear will be lower than front brakes. This will prevent premature rear wheel lock-up and skid. If hydraulic pressure is lost in one half of front to rear split system, operation of remaining half of brakes hydraulic system is not effected. There is only one proportioning valve used on this vehicle. During any service procedures identify combination valve used in vehicle to replacement combination valve by part number as well as split point (PSI) and slope.