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Summary
  • The brake proportioning valve is a spring-loaded device that improves brake balance by reducing rear brake pressure during hard stops.
  • The brake proportioning valve only works during harsh braking conditions when hydraulic pressure exceeds the minimum level or the “split point.”
  • Some symptoms of a faulty brake proportioning valve include rapid rear brake wear, rear-wheel lockup or skidding, and increased braking distance.

The braking system is composed of some of the most essential safety components of your vehicle, which is why making sure every part of it works should be one of your top priorities.

In this article, we’ll talk about the brake proportioning valve.

Brake Proportioning Valve on white background
The brake proportioning valve is a spring-loaded device that improves brake balance by reducing rear brake pressure during hard stops.

What is a Brake Proportioning Valve?

The brake proportioning valve is a spring-loaded device that improves brake balance by reducing rear brake pressure during hard stops. It is typically found on or near the master cylinder of most older vehicles.

brake proportioning valve diagram
Brake proportioning valve diagram | Image source: Richard McCuistian

How Does a Brake Proportioning Valve Work?

Under normal braking conditions such as light or moderate braking, the proportioning valve stays open, allowing equal hydraulic pressure to be supplied to both front and rear brakes. This is because light braking does not allow enough weight to be transferred from the rear to the front of the vehicle, which means rear-wheel lockup is less likely to occur.

The brake proportioning valve only works during harsh braking conditions when hydraulic pressure exceeds the minimum level or the “split point.” Under this circumstance, a piston attached to the rear brake circuit forces the valve to close, restricting hydraulic pressure from the rear brakes by 20-40%. This allows the front brakes to have higher hydraulic pressure than the rear, which prevents the vehicle from skidding during a panic stop.

Once you release the brake pedal, a spring pushes the piston that’s blocking the valve to one side, allowing hydraulic fluid to pass through the front and rear brakes equally.

Height-Sensing Proportioning Valves

A height-sensing or load-sensing proportioning valve is typically used in vehicles that carry heavy cargo, such as vans and pickup trucks. It works a bit differently from the conventional mechanical valve. Instead of restricting pressure, this valve adds more hydraulic pressure to the rear brakes to generate greater braking force than the front.

height sensing proportioning valve diagram
Height sensing proportioning valve diagram | Image source: Richard McCuistian

As its name suggests, the height-sensing proportioning valve works by adjusting rear brake pressure depending on the vehicle’s height. For instance, if a truck’s cargo area is heavily loaded, its rear end will typically drop. This will cause the lever on the proportioning valve to work harder by supplying greater hydraulic pressure to the rear brakes compared to when the vehicle is lightly loaded.

Height-Sensing Proportioning Valve Adjustment

If your vehicle has a height-sensing proportioning valve, note that replacing parts such as air lift shock absorbers or springs may affect its performance, resulting in reduced stopping distances. When replacing such components, make sure to consult your mechanic to check if the new part is compatible with your vehicle’s specifications.

In case the valve itself is in need of a replacement, make sure to adjust its settings according to your vehicle’s height. This ensures that proper pressure will be applied to the rear brakes in the event of a panic stop.

Electronic Brake Proportioning

Most late-model vehicles have eliminated the need for mechanical brake proportioning valves. Instead, they are equipped with electronic brake proportioning or electronic brake distribution (EBD), which is controlled by the anti-lock braking system (ABS).

To maintain brake balance, the ABS monitors wheel speed using wheel speed sensors (WSS). If it detects that both the front and rear wheels are slowing down at a similar rate, the rear wheels will continue to receive the same braking force as the ones in front.

Once the rear wheels start to lock up, the ABS will reduce pressure from the rear brakes by closing the solenoids in the hydraulic unit.

abs unit module control box
Most late-model vehicles have eliminated the need for mechanical brake proportioning valves.

Symptoms of a Bad Brake Proportioning Valve

A brake proportioning valve typically fails due to corrosion, just like any other metal vehicle component. When it fails, the rear brakes may not receive any pressure at all. If you see rusty rear rotors on a vehicle that is driven every day, the proportioning valve may be preventing fluid pressure from reaching the rear brake assemblies.

On the other hand, the brake pressure may increase too rapidly, causing the vehicle to become unstable under heavy braking; on curves, this can lead to disaster. Keep an eye out for the following signs that may indicate a bad brake proportioning valve:

Rear-Wheel Lockup or Skidding

A faulty brake proportioning valve may cause the vehicle’s rear wheels to lock up, resulting in a skidding movement as tires lose traction.

Rapid Rear Brake Wear

The proportioning valve limits brake pressure from the rear wheels, which shouldn’t allow the rear brakes to wear faster than the front. If the rear brakes wear out more quickly than normal, have a mechanic check your vehicle for a damaged brake proportioning valve.

mechanic inspecting brakes
Have a mechanic check your vehicle for a damaged brake proportioning valve if the rear brakes are wearing out more quickly than normal.

Increased Braking Distance

A faulty brake proportioning valve could increase your vehicle’s braking distance, as it might reduce the pressure from the rear brakes too much.

Note that the symptoms of a bad brake proportioning valve may also point to damage to other components, such as the rear brake circuit, brake caliper, or wheel cylinder. If you notice any of these symptoms, it’s best to have the entire braking system checked to ensure that no other parts are causing problems.

How to Check Your Brake Proportioning Valve

f you’re an avid DIYer, there are various procedures you can do to test your vehicle’s brake proportioning valve.

A simple test can be done using two pressure gauges attached to the brake line from the master cylinder and the rear brake outlet of the proportioning valve. When the brake pedal is depressed with enough force to reach the split point, the gauge connected to the rear brakes should increase at a slower rate than the gauge connected to the master cylinder. If this doesn’t happen, the proportioning valve may be defective.

If you’re not confident with your DIY skills to perform the test, however, it’s always a good idea to have a professional check your vehicle to ensure a proper diagnosis.

Do Drum Brakes Need a Proportioning Valve?

Yes. Vehicles with front disc and rear drum brakes need a proportioning valve (or two) to achieve optimal braking performance, which is when the front brakes lock up before the rear brakes.

3 Basic Kinds of Valves in a Non-ABS System

To put this discussion about brake proportioning valves in context, there are actually three basic kinds of valves in play in a non-ABS system.

  • Residual valves are used to maintain constant pressure on the brake system. For drum brakes, a 10 lb. residual valve is used to hold pressure against the springs in the drum. This allows the drum brake to engage more quickly and reduces pedal travel.
  • A metering valve will control fluid flow from the master cylinder causing the rear drum brakes to engage just before the front disc brakes. This allows the car to settle evenly under braking instead of nose diving.
  • A proportioning valve prevents rear wheel lockup during hard braking/panic stops. In many cases these are in the line leading to the rear brakes and there will be one on each side. This is important, particularly when a panic stop happens while going into a road curve and tends to reduce spinout.

Some systems have these valves combined in one unit.

non abs system valve diagram
Diagram showing valves in a non-ABS system | Image source: Richard McCuistian

Where to Get a New Brake Proportioning Valve for Your Vehicle

Avoid driving with a faulty brake proportioning valve because it can cause braking issues and increase the risk of accidents. Luckily, you can get a high-quality replacement without breaking the bank at CarParts.com with just a few clicks.

You don’t even have to leave the comfort of your home to find and order what you need because you can browse our catalog on your mobile device or computer. Use our vehicle selector to narrow down the results to the valves that fit your ride, then adjust the filters based on your preferred brand, price, and feature.

Carefully handpicked by a team of professionals, our brake proportioning valves are built to last. They’re on hand, and we ship orders immediately from our strategically located distribution centers across the country. So you can get your new part and repair your ride in no time.

Check out our catalog and shop now!

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About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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