DIY

Brake Proportioning Valve Problems

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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 a critical part of many braking systems⁠—the proportioning valve. Read on to find out what it is and what problems your vehicle may experience if it fails.

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, simply known as the 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.

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.

Replacement Brake Proportioning Valve
The brake proportioning valve only works during harsh braking conditions when hydraulic pressure exceeds the minimum level or the “split point.”

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.

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 brake pressure increases rapidly, causing the vehicle to become unstable under heavy braking. 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

If 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.

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