Brake Valve Discussion (Proportioning, Metering and Combination Valves)

Visit our website for more resources geared towards technicians, DIYers and automobile enthusiasts:

Ever wonder how balance is achieved in your braking system. You have most likely heard of proportioning valves, combination valves and metering valves. But do you know what these valves do? In this training video, we discuss the evolution of brake valves over the years.

Visit our Website: http://www.raybestos.com
Like us on Facebook:   / raybestosbrandbrakes  
Follow us on Twitter:   / raybestosbrakes  
Follow us on Instagram:   / raybestos_brand  
Follow us on LinkedIn:   / raybestos-brake  

Combination Valve

Before the turn of the century, vehicles with disc brakes in the front and drum brakes in the rear had a combination valve. Combination valves are commonly called proportioning valves. This valve is typically located below the master cylinder. The combination valve is a spring-operated device that has three key components:

1.) Metering Valve: The metering valve is located in-line with the front brakes. It holds off pressure to the front calipers until the rear brakes build up enough pressure. Once enough pressure builds up in the system (75-125 psi), the metering valve opens and allows the fluid to flow to the front calipers. Without the metering valve, the disc brakes would apply much quicker than the drum brakes. This would cause a nose dive during braking.

2.) Proportioning Valve: The proportioning valve is located in-line with the rear brakes. This valve prevents rear brake lockup during sudden and hard stops. This valve remains open until a panic braking situation. During sudden braking, high pressure from the master cylinder causes the proportioning valve to close. This restricts fluid pressure to the rear brakes and prevents them from locking up.

3.) Pressure Differential Switch: The pressure differential switch is located in the center of the combination valve. If the brake system loses pressure in the front or rear portion of the brake system, the switch grounds. This illuminates the warning light on the dashboard.


Height-Sensing Proportioning Valve

A height-sensing proportioning valve is found on many light trucks with disc brakes in the front and drum brakes in the rear. It's located in the hydraulic line leading to the rear drums.

Height-sensing proportioning incorporates a lever that is attached to the axle. As the load in the truck increases or decreases the lever moves the proportioning valve. If the load increases, the lever opens the valve to allow more pressure to the rear brakes. If the load decreases, the lever closes the valve to decrease pressure to the rear brakes.

Dynamic Rear Proportioning

Dynamic rear proportioning is the standard in late model vehicles. This electronic system is integrated into the ABS system. As you brake, the system monitors deceleration rates for each wheel. The system then reduces pressure at the rear wheels before they reach the point of lock up.

Since this system is dynamic, it is constantly monitoring and changing the brake pressure to the front and rear brakes. This provides optimum braking.