Brake Proportioning/Combination Valve: Description and Operation

Fig. 8 Combination Valve:

Fig. 9 Diagonally Split System Combination Valve:

The combination valve, Fig. 8, is a metering valve, failure warning switch, and a proportioner in one assembly and is used on disc brake applications. The metering valve delays front disc braking until the rear drum brake shoes contact the drum. The failure warning switch is actuated in event of front or rear brake system failure, in turn activating a dash warning lamp. The proportioner balances front to rear braking action during rapid deceleration.
Combination valves used on diagonally split brake systems do not use metering valves and use two proportioning valves, Fig. 9.

Fig. 11 Metering Valve, Continued Braking:

Fig. 10 Metering Valve, Initial Braking:

Fig. 12 Three-way Combination Valve:

Fig. 13 Two-way Combination Valve:

Metering Valve
When the brakes are not applied, the metering valve permits the brake fluid to flow through the valve, thus allowing the fluid to expand and contract with temperature changes.
When the brakes are initially applied, the metering valve stem moves to the left, preventing fluid to flow through the valve to the front disc brakes. This is accomplished by the smooth end of the metering valve stem contacting the metering valve seal lip at 4 to 30 PSI, Fig. 10. The metering valve spring holds the retainer against the seal until a predetermined pressure is produced at the valve inlet port which overcomes the spring pressure and permits hydraulic pressure to actuate the front disc brakes, Fig. 11. The increased pressure into the valve is metered through the valve seal, to the front disc brakes, producing an increased force on the diaphragm. The diaphragm then pulls the pin, in turn pulling the retainer and reduces the spring pressure on the metering valve seal. Eventually, the pressure reaches a point at which the spring is pulled away by the diaphragm pin and retainer, leaving the metering valve unrestricted, permitting full pressure to pass through the metering valve.
On some late model applications, either two-way or three-way combination valves are used. The three-way combination valve consists of a metering valve, failure warning switch and a proportioner mounted in an aluminum body, Fig. 12. The two-way combination valve, Fig. 13, consists of a failure warning switch and a proportioner. On models equipped with metering valves, the metering valve release rod must be pushed in during brake bleeding operations on the front wheels.

Fig. 14 Failure Warning Switch, Rear System Failure:

Failure Warning Switch
If the rear brake system fails, the front system pressure forces the switch piston to the right, Fig. 14. The switch pin is then forced up into the switch, completing the electrical circuit and activating the dash warning lamp.
When repairs are made and pressure returns to the system, the piston moves to the left, resetting the switch. The detent on the piston requires approximately 100 to 450 PSI to permit full reset of the piston. In event of front brake system failure, the piston moves to the left and the same sequence of events is followed as for rear system failure except the piston resets to the right.

Fig. 15 Proportioner, Rapid Deceleration:

Proportioner or Pressure Control Valves
During rapid deceleration, a portion of vehicle weight is transferred to the front wheels. This resultant loss of weight at rear wheels must be compensated for to avoid early rear wheel skid. The proportioner or pressure control valve reduces rear brake system pressure, delaying rear wheel skid. When the proportioner or pressure control valve is incorporated in the combination valve assembly, pressure developed within the valve acts against the large end of the piston, overcoming the spring pressure, moving the piston left, Fig. 15. The piston then contacts the stem seat and restricts line pressure through the valve.

Fig. 9 Diagonally Split System Combination Valve:

Fig. 15 Proportioner, Rapid Deceleration:

Fig. 17 Pressure control valves installed in master cylinder. Late 1983 & 1984---87 front wheel drive models:

During normal braking operation, the proportioner or pressure control valve is not functional. Brake fluid flows into the proportioner or pressure control valve between the piston center hole and the valve stem, through the stop plate and to the rear brakes. Spring pressure loads the piston during normal braking, causing it to rest against the stop plate, Fig. 16.
On diagonally split brake systems, two proportioners or pressure control valves are used. One controls the left rear brake, the other the right rear brake. On exc. late 1983 and 1984-87 front wheel drive models, the proportioners or pressure control valves are located in the combination valve, Fig. 9. On late 1983 and 1984-87 front wheel drive models with power brakes, the proportioners or pressure control valves are installed in the master cylinder rear brake outlet ports, Fig. 17.