A proportional valve is a component of the brake system. It’s designed to adjust hydraulic brake force to the rear brakes proportional to the expected weight on the rear axle.
Brake system proportional valves have an important job; they are tasked with automatically adjusting the front-to-rear brake force bias or balance for optimal braking under all conditions.
On this page, we’ll cover what a proportional valve is, why it’s essential and where it is located on a car.
Why Proportional Valves are Important
A vehicle typically nose dives at the front under braking, which causes the vehicle’s weight to shift about, making the front axle weigh heavier and the rear axle lighter. Under these conditions sending equal brake force to both axles will cause the rear wheels to lock up. Not ideal, especially under slippery road conditions.
The obvious solution, then, is a valve that can automatically and quickly adjust brake force under these conditions, and that’s exactly what the proportional valve does.
There are a few different types, and that’s what we’ll look at next.
Proportional Valve Types
As brake systems were originally purely mechanical systems, so too was the original proportional valve. As the brake systems have evolved, we now have some help from electronics.
The three most common types are as follows:
- Mechanical load sensing valve
- Combination metering valve
- ABS-controlled brake bias
Mechanical Load Sensing
The mechanical load sensing valve is fitted on the chassis next to the rear axle. As the suspension height changes under braking and when the vehicle is loaded, a lever from the valve to the suspension deflects the valve and, therefore, the rear brake force.
It’s a simple design but suffers from corrosion, especially in salt states.
The combination valve is so called because it served three functions – it contained a proportional valve, a metering valve, and a brake circuit pressure loss warning sensor. This old technology won’t be fitted to modern passenger vehicles today.
We already know what the proportional valve part does and adjusts front and rear brake pressure to optimize brake force under braking with different vehicle loads.
The metering valve is required for vehicles with rotors to the front and drums brakes to the rear. Brake pads rest at the rotors, ready for action, but not for drum brake shoes. They require brake pressure earlier than brake pads to prime for action, and that is what the metering valve does; it allows rear brake pressure to build up before the front, just enough, so they bite at the same time.
The pressure differential switch monitored brake circuit pressure and illuminated a dash warning light if it detected a brake system leak.
ABS Controlled Brake Bias
We all know what ABS does; its sole job is to prevent wheel lock-up by electronically controlling hydraulic brake force to each wheel. And because ABS can do this so quickly and accurately makes sense, it also takes on the task of optimizing brake force from to rear electronically.
And so ABS-controlled brake bias isn’t a proportional valve at all; the valve has been eliminated or absorbed into an intelligent brake system that is capable of a ton of other safety systems that are still evolving.
Where’s a Proportional Valve Located?
As you have learned, modern cars don’t have a proportional valve, the function of the valve has been absorbed by the ABS, and it is located on most vehicles under the hood close to the brake fluid reservoir.
You can easily identify the ABS unit as it sports several hard brake lines and a large electrical terminal connector.
The old-school load sensing valve used on vehicles of the 1990s is located on the rear chassis close to the rear axle.
The combination valve is typically located close to the brake master cylinder under the hood.
You may find these posts helpful:
- About the Author
- Latest Posts
John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.