Combination/Proportioning Valve

DESCRIPTION






The combination valve is attached to the frame rail below the master cylinder. It has five fluid ports and a brake lamp warning switch on its exterior. Internally, it has a pressure differential valve and a single proportioning valve.






The combination valve is used for controlling and monitoring the hydraulic portion of the brake system. This is accomplished by combining a pressure differential valve, a warning light switch with a single proportioning valve into one unit.

OPERATION
As stated earlier, the combination valve has five fluid ports on it. Two of the ports receive fluid from the master cylinder and the other three supply fluid to the brakes. The brake fluid coming from the primary port on the master cylinder is dispersed to each front brake by the combination valve. The brake fluid coming from the secondary port on the master cylinder is dispersed to a single brake tube leading toward the rear brakes where it is split to the individual brakes.
To best describe the operation of the combination valve and how it controls the brake fluid while it is inside the valve, it is best break the valve into separate topics.

PRESSURE DIFFERENTIAL VALVE AND WARNING LAMP SWITCH
The pressure differential valve and warning lamp switch is part of the combination valve to alert the driver in the event pressure drops in one of the brake hydraulic systems. The function of the pressure differential switch and warning lamp switch are described in detail below.
The brake hydraulic system on this vehicle is vertically split. This means the brake hydraulic system is equally split between the front and rear brakes, with both front brakes on one hydraulic system and both rear brakes on the other. This differs from the brake hydraulic system on a front wheel drive car which is diagonally split. On a brake hydraulic system which is diagonally split, the left front and right rear brake is on one hydraulic system and the right front brake and left rear brake on the other.
The primary and secondary hydraulic circuits from the master cylinder are kept separated into a vertically split hydraulic circuit by the internal passages of the combination valve.
The pressure differential valve, which is internal to the combination valve, is used to monitor the pressure in the brake hydraulic system. The pressure differential valve functions by simultaneously sensing that the hydraulic pressure in the primary and secondary hydraulic circuit from the master cylinder and the front and rear brake hydraulic circuits is equal.
If the pressure in the brake hydraulic circuits is equal when the brake pedal is depressed, the differential valve will stay centered in the combination valve and the red BRAKE warning lamp will not be turned on. If pressure in one of the brake hydraulic circuits drops when the brake pedal is depressed, the drop in pressure will cause the pressure differential valve to move off center in the combination valve. As the pressure differential valve moves off center it pushes the plunger up into the brake warning lamp switch. When the plunger is pushed into the brake warning lamp switch, a ground for the red BRAKE warning lamp circuit is completed. Creating the ground for the red BRAKE warning lamp circuit will turn on the red BRAKE warning lamp located in the instrument cluster.
If the red BRAKE warning lamp is turned on, the brake system requires immediate service. However, since the brake hydraulic systems are split front to rear, the vehicle will retain half of its stopping capability in the event of a failure in either half of the system.

NOTE: The brake warning lamp switch is the latching type. It will automatically center itself only after the repair is made and the brake pedal is depressed.

PROPORTIONING VALVE






The proportioning valve balances front to rear braking by controlling (at a given ratio) the increase in rear brake system hydraulic pressure above a preset level. The proportioning valve section operates by transmitting full input pressure to the rear brakes up to a certain point. This is called the split point. Beyond this point it reduces the amount of pressure increase to the rear brakes according to a certain ratio.
On light pedal applications equal brake pressure will be transmitted to the front and rear brakes. On heavier pedal applications the pressure transmitted to the rear will be lower than the front brakes. This will prevent premature rear wheel lock-up and skid. If hydraulic pressure is lost in one half of the front to rear split system, the operation of the remaining half of the brakes hydraulic system is not effected.
There is only one proportioning valve used on this vehicle. During any service procedures identify the combination valve used in the vehicle to the replacement combination valve by part number as well as split point (PSI) and slope.