Suburban 3/4 Ton 4WD V8-305 5.0L (1986)
Brake Proportioning/Combination Valve: Description and Operation
These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to
the combination valve.
The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this
valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic
fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure
warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system.
The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and
brake failure warning switch operate the same as those in the two function combination valve previously described.
The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing
rear wheel lock-up.
The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and
responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle.
