Brake Proportioning Valve Reset | Symptoms, Bleeders, Adjustment
Brake Bias and Proportioning Valves
When you hit the brakes, the master cylinder sends high pressure brake fluid to the brake calipers/wheel cylinders to engage the brake pads/shoes. You may not realize it, but which brakes are used the most changes based on how much pressure is applied to the brake pedal. At low speeds, there is little weight transfer rear to front, so the rear brakes engage first, slowing the vehicle. This saves the front brakes from premature wear. As you press harder on the brakes, the pressure sent to the rear brakes is scaled down, while the front brakes get full pressure. In heavy braking or a panic stop, there is considerable weight transfer to the front of the vehicle, reducing the rear brake’s ability to slow the vehicle without locking up. This balance is called brake bias and is managed by the vehicle’s proportioning valve.
What Does a Proportioning Valve Do?
So just what is a proportioning valve and what does it do? A proportioning valve allows all the brake fluid to pass through normally under typical braking. When the pressure threshold is reached, a spring-loaded piston moves, closing off the rear brake port in the valve. At this point, all braking pressure is directed to a smaller piston, which allows the rear brake lines to continue to be pressurized, but at a lesser rate. Because of how these valves work, it is possible for the valves to trap air within them that yields spongy brakes as well as become “off-set,” sending the wrong pressure to the rear brakes. Note that a proportioning valve can be fixed or adjustable, or part of a combination valve, but is a separate component of the braking system. There is no such thing as a “proportioning valve master cylinder” for example.
For clarification, a stock proportioning valve is much more than that, the correct term is “combination valve,” as it does multiple things. First, it distributes the brake fluid to each corner of the vehicle via the brake hard and soft lines. Second, it handles the proportioning duties (often with a metering valve in the front brake system), but there is a third feature—a safety valve that cuts off fluid flow in the event of a catastrophic line failure where there is no pressure. If this failure occurs all fluid is shut off from that half of the system, so that the other half still works and allows you to safely stop the car. Not all OE brake line blocks have proportioning valves, many are just distribution blocks, especially on older four-wheel-drum brake equipped rides.
The balance of braking bias front to rear varies by the placement of the drive wheels, RWD vehicles typically use the front brakes for about 60 percent of the braking, while FWD vehicle front brakes perform about 80 percent of the braking. This is why the rear brakes are smaller than the fronts. Brake bias is all about maintaining vehicle control under braking. The idea behind brake bias is for each end of the system (front and rear) to reach their maximum deceleration force at the same time to minimize the braking distance and reduce skidding. While the tire’s contact patch with the road surface is technically the limiting factor for all braking, the bias plays a vital role in reducing tire skid during braking. If the rears lock up first, you may lose control of the vehicle and can end up in a ditch or worse. Brake bias is meant to reduce/eliminate rear lock-up before the fronts.
There are a few ways to design brake systems to maintain the proper bias, however the main device used to ensure this balance is a proportioning valve. How this works is quite complicated, it is more than just a simple control valve. Up to a certain pressure, the valve allows both front and rear brakes to receive equal pressure. Once the pressure exceeds that point, the rear brake pressure is allowed to increase at a slower rate than the fronts, ensuring the rear brakes don’t lock up.
Adjustable Proportioning Valve VS Non-Adjustable Proportioning Valve
While most OE valves are fixed (non-adjustable), many aftermarket proportioning valves are adjustable. Unless you have modified your braking system or added/reduced weight that will affect the bias, there is no need to adjust the valve, it is set from the factory for a stock vehicle. Once you make changes though, the bias can be wildly different, which is why it is important to evaluate your braking system. Proportioning valve adjustment requires a lot of trial-and-error testing to check the function. If your rear brakes tend to lock up before the fronts at any speed, you need to adjust the valve. Many brake kits use a stock valve because they are closely matched to the factory specs, but when you upgrade to multi-piston brakes or larger brakes, front and rear bias can be radically altered, making an adjustable valve much more important.
Brake Proportioning Valve Diagram
How to Reset Proportioning Valve
If your proportioning valve needs to be reset, there are a couple of ways to perform a proportioning valve reset. This happens when you bleed the brakes, and the pressure differential valve trips due to the pressure differential in the system when bleeding. In the event of a loss of brake pressure, for example a brake line splits, the proportioning valve’s pressure differential valve trips, which stops all fluid from passing through the open line, giving you the ability to stop on the other half of the braking system. This valve can get tripped when you are manually pedal bleeding the brakes, as the valve sees the brake bleeding process as a hydraulic line failure. You know that this occurs when you get no fluid through the brake bleeder even though the master is full, and the pedal is depressed. In such a case a proportioning valve reset is needed to have both front and rear brake circuits functioning properly again (and to turn off the brake warning light on the dash, if equipped).
To test the valve, you can use a standard automotive 12v test light on the warning light switch. Connect the test light’s clamp to the battery 12v positive post, and then touch the probe to the switch. If the test light illuminates the valve is tripped. If the test light does not light up the valve is in the normal position. Leave the test light connected and see the steps below on how to reset proportioning valve options:
In most cases, the valve will self-correct once the pressures equal out on both sides (front and rear). With the brake system closed and the reservoir full, start the engine. The running engine will provide brake assistance to make this easier (on power brake systems). Press the brake pedal three times, with even, gentle pressure, until the pedal is applied as far as it can go. That should recenter the valve and your proportioning valve reset will be complete.
If the valve still does not reset, open one bleeder on the side of the valve that IS getting brake fluid. Have an assistant slowly press the brake pedal until the test light turns off. As soon as the test light turns off close the open brake bleeder, the valve is now reset/centered, and you should be good to go.
If the above options do not work, then you could have a damaged valve. Rust, contaminants, and other issues can cause the valve to stick. You can manually reset the valve by removing the brake lines, along with the warning switch. Inside the warning switch port, you can see the pressure differential valve, a rod like piece of brass in the prop valve body. It should neck down on both sides to a smaller diameter center groove. If the valve is tripped, the rod will look like a smooth shaft with no necked down area visible through the port that the warning light is usually threaded into. You can use a pick tool to move the rod back into the center position. This method works, but if you must do this, your valve is likely damaged and needs to be replaced.
Symptoms Of a Bad Proportioning Valve
If you have spongy brakes, must pump the pedal to get a firm pedal, or the front/rear brakes are locking up, you may have a tripped or faulty prop valve. If the pressure differential valve has tripped, one half of the system will not get any fluid. If the valve is damaged or clogged up with rust or contaminants, the pressures will not be equalized, causing unstable braking performance. You may need to perform a proportioning valve reset, as described above, or replace the valve if faulty.
How to Bleed a Brake System with A Proportioning Valve You can avoid the reset issue altogether if you bleed your brakes differently. There are three methods to bleed brakes- manual pedal actuation (the most used), vacuum bleeding, and pressure bleeding.
Manual pedal actuation is done with two people, each bleeder is opened one at a time from the furthest to closest to the master cylinder. Each caliper/wheel cylinder is bled until there is no air in the line and closed. This type of bleeding is easy, requires only a wrench, and is also the most likely to trip the safety valve. You can stop the valve from tripping by using a proportioning valve bleeder tool. This tool replaces the warning switch temporarily until the bleeding is finished.
Vacuum bleeding uses a small vacuum pump, usually hand-operated, and connects to each bleeder valve one at a time to pull fluid and air through the system without operating the pedal. This works well and can be done by one person.
Pressure bleeding is an even better option. You can pressure bleed from the top side (master cylinder) or bottom (caliper/wheel cylinder side). Top side pressure requires a bleeder with adapter caps to seal to the master cylinder, but bottom-side pressure using a reverse bleeder forces fluid from the caliper/wheel cylinder up to the master, eliminating all air. This type of bleeding works well and eliminates the need to bench-bleed your master cylinder.
Modifications That Cause Proportioning Valve Failure
You can certainly add upgrades to your vehicle’s braking system that affect the brake bias. Using the wrong brake fluid (glycol-based vs silicone fluid) can cause the internal components to rust very quickly, locking them up and preventing the proportional valve working as it should. Old brake fluid contains a lot of water (brake fluid is hygroscopic, it absorbs water naturally), leading to rust and corrosion. However, if you regularly flush and bleed your brake system and the brake system is otherwise sized correctly and working properly, there is should be no need to replace your stock prop valve or upgrade to an adjustable proportioning valve.
Physical upgrades, such as larger rotors, bigger/more aggressive pads, or piston count will increase the front bias. Any reduction in rear braking also shifts the bias forward. Even lowering your vehicle or adding weight over the rear axle shifts the brake bias forward, putting more strain on the prop valve, reducing its efficiency.
In the rear, increasing brake size, adding sticky tires, and more aggressive brake pads/shoes, shifts the bias rearward. Lifted suspension raises the center of gravity, moving the bias rearward as well. Weight reduction in the rear or adding weight in the front (such as a heavier engine) also shifts the weight forward, affecting rear bias. Any of these situations may call for an adjustable proportioning valve to properly move the brake bias to the front or rear of the vehicle as needed for these modifications.
Adjustable Proportioning Valve Installation
When you start modifying your vehicle, adding an adjustable brake proportioning valve gives you the ability to change the brake bias to better suit your individual situation. As mentioned above, a lot of seemingly unrelated factors play into brake bias, so once you stray from stock, you can quickly upset the entire braking dynamics of the vehicle.
When adding an adjustable valve, you need to remove the factory unit. Not all vehicles have a prop valve, so verify before changing parts. You need to convert to a basic distribution block to eliminate a factory valve, or in some cases you can “gut” the factory valve by removing the internal components and thus turning it into a distribution block. If this is possible for your factory valve it does save some time having to make new brake lines. The latest product offering is an adjustable valve built right into a distribution block. The aftermarket valve can be installed anywhere in the rear brake line. Under the hood is nice, as you can access it more easily. If you opt for a valve with a gauge, you can see the actual pressure being put on the rear brakes. In all out racing applications, some builders even run the rear brake line along the transmission tunnel inside the car, so the adjustable valve is within the driver’s reach for on-the-fly bias adjustments to counter dropping fuel levels, tire wear, or other track conditions.
Adjusting Brake Proportioning Valve The process for brake proportioning valve adjustment is simple, but it does require some trial and error. Before any testing, make sure you are in a remote area, preferably a large empty parking lot. Your tires should be in good condition and properly inflated. The vehicle should be relatively empty, you don’t want stuff flying around when you are panic stopping. That said, you do want the vehicle to be at normal ride height and weight. For most applications that will be the driver with a half tank of gas.
The basic process is to install the valve, rotate the knob to the lowest setting, and then drive the car. Attempt several hard stops at 30 mph. Have an assistant watch the vehicle, looking for wheel lock up. Increase the rear bias adjustment and drive again until the front and rear wheels both lock up at the same time. Once this point has been found, repeat the test at 50-60 mph. The front and rear wheels should lock up at the same time. It can take a few tries, but you don’t max it out to start, as this can reduce the rear brake effort too much. Properly adjusting your brake bias this way is especially important on staggered wheel/tire packages, major brake system updates, or any changes to the vehicle’s weight bias (setting the engine back with adjustable motor mounts for example). Adjusting brake proportioning valve steps take a little time, but the benefits of optimized braking for your vehicle’s weight, brake system, and tire package are worth the effort.
Your brakes are the single most important feature your vehicle has, do not play games with the system. Far too often the braking system is ignored after making significant changes to the suspension and drivetrain, leaving you with a woefully unbalanced and under-performing braking system. Make sure you can stop your car when you need it to stop by understanding brake systems and updating the brake bias to match your vehicle’s needs.
Proportioning Valve Bleeder Tool
Though mentioned earlier in our section on brake bleeding options, we felt the use of a proportioning valve bleeder tool should be explained in more detail. The typical GM-style proportioning valve that a lot of aftermarket brake upgrade kits use features a brake warning light switch mounted on the top of the proportioning valve. This switch illuminates the “brake” warning light on your dashboard of most cars built from 1968 on up when the brake proportioning valve detects a pressure loss in the front or rear brake system. It does this by using brake pressure from the still working front or rear system to push the pressure differential valve fore or aft in the valve body (reference the brake proportioning valve diagram above).
This valve has a machined center that sits directly under the light switch, allowing the light switch’s plunger to remain extended. During a failure, the differential valve moves and depresses the light switch plunger, illuminating the dash warning light. While many custom cars do not use stock instruments/warning lights, you could wire in a warning light if you so choose.
The issue here when servicing your brake system with a GM-style proportioning valve in the system is the possibility of accidentally tripping the pressure differential valve inside. The proportioning valve bleeder tool we offer is essentially a dummy switch that you temporarily install in place of the functioning brake warning light switch. This tool/dummy switch has a fixed plunger body on the bottom of the tool, which holds the pressure differential valve in place during standard manual brake bleeding operations. It is a must have tool in your brake bleeding arsenal to make bleeding most any brake system quick and easy that uses a GM-style proportioning valve.
