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Residual vs Proportioning Valves and What Your Brake System Needs

1/26/2021
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Make the Most of Your Brake System

Typically, we focus on performance and looks when it comes to a muscle car or street rod build. We want lots of horsepower and neck-breaking paint and body with the perfect stance sitting on the perfect set of wheels. There is nothing wrong with big digit horsepower (hey, who does not love a good smokey burnout?) and a paint job that looks six miles deep, but when people ask us about upgrades for their project one of the most often under appreciated category is braking. Often, we see people using stock brakes with larger wheel and tire packages and horsepower that is usually double or triple what the car left the factory with. As such, we will always recommend brake upgrades be part of the plan.

Converting to disc brakes should be considered a must with today’s traffic and driving speeds and adding power brakes makes for a much easier driving car. That said, no one brake solution is going to fit every project outline. Installing power brakes in an original 32 Ford frame versus adding power brakes in a ’69 Chevelle are quite different. You must consider things like front and rear weight bias, tire sizes (the contact patch of the tire determines the braking effectiveness of your brake system), caliper and wheel cylinder capacity, and more. Then “dial” your brake system in with the properly sized power brake booster, disc brake rotor diameter, and caliper size/number of pistons. The brake torque can be further fine-tuned by utilizing and adjustable brake proportioning valve. For applications where the master cylinder is lower than the calipers/wheel cylinders, residual pressure valves must be utilized to maintain a firm pedal as well.

What is a Residual Pressure Valve?

A residual pressure valve (abbreviated RPV) will hold a small amount of fluid pressure in the brake system when the brake pedal is in the fully retracted position. The purpose of this is to eliminate the possibility of fluid transferring from the caliper or wheel cylinder back into the master cylinder reservoir. This problem is extremely common when the master cylinder is mounted low in the vehicle such as with frame-mounted under floor master cylinder locations. When the master cylinder is lower than the brake calipers or wheel cylinders brake fluid will naturally want to gravity feed from the higher caliper/wheel cylinder location down to the master cylinder reservoir. To stop this from happening we use a residual pressure valve, which is like a residual check valve and has a spring-loaded ball and seat inside it. The RPV will allow fluid to flow to the caliper or wheel cylinder when the brake pedal is depressed. When the brake pedal is released the RPV allows brake fluid to return to the master cylinder reservoir until the pressure downstream of the valve drops to its pressure rating of 2 or 10 psi. At that point, the valve will close off and prevent any further fluid transfer back to the master cylinder. This helps maintain a firm, ready brake pedal, but the pressure is not high enough to affect the braking system and cause pad/shoe drag.

Which Residual Pressure Valve Should I Install, 2 psi or 10 psi?

The two commonly used RPV pressure ratings are 2 psi and 10 psi. The 2-psi valve is used for disc brake calipers and the 10-psi valve is used for drum brakes with wheel cylinders. The reason for the two different pressure ratings is due to the amount of pressure required to initiate braking in the two different brake systems. Drum brakes have a series of springs with tension against the wheel cylinder which needs to be overcome before the brake shoes will contact the drum and begin to apply. Also, the piston size of a drum brake wheel cylinder is very small, making it less sensitive to small amounts of fluid pressure and is why we suggest a 10-psi valve for drum brakes. Disc brakes, on the other hand, have calipers with large piston sizes and very little resistance to movement. The only resistance to movement in a disc brake caliper is the O-ring seal around the piston that aids in retracting the piston just enough to prevent the pad rubbing on the rotor surface. The large piston size and low resistance to piston movement makes disc brakes very sensitive to small amounts of fluid pressure. Even as little as 5- or 10-psi of fluid pressure can cause them to drag so always be sure to use 2-psi residual valves with disc brakes.

In applications where there are disc brakes in the front and drum brakes in the rear it will require one 2-psi RPV for the front and one 10-psi for the rear. Four-wheel disc brake systems will use two 2-psi valves. Four-wheel drum systems will typically require two 10-psi valves, however if an early style single outlet master cylinder is used then a single 10 psi residual valve can be used if it is installed before the T fitting which splits off to the front and rear brakes. The typical brake residual valve location is along the frame rail of the vehicle where there is a nice straight section of brake line to add it to. However, any area where there is enough line length for the installation is suitable, including in the engine compartment.

Do I need a Residual Pressure Valve with a Firewall Mounted Master Cylinder?

Firewall mounted master cylinders, which are located higher than the brake calipers or wheel cylinders, do not have issues with fluid flowing back into master cylinder reservoir like low mounted master cylinders do. For this reason, residual pressure valves are not mandatory, however they can still be used. For instance, drum brake systems may result in a higher and firmer pedal feel when using a 10-psi residual valve compared to none at all. Disc brake systems typically do not have a noticeable difference in function with or without a RPV when the master cylinder is mounted up high on the firewall.

What Does a Proportioning Valve Do?

The function of a proportioning valve is to supply the proper amount of fluid pressure to the front and rear brakes to make the vehicle stop quickly and safely. If there is not enough fluid pressure to the rear brakes compared to the front, your vehicle will not stop as quickly as it could. If there is too much fluid pressure to the rear brakes, then the rear will lock up too early and make the vehicle very unstable under hard braking and may cause the driver to lose control. The rear end will want to step out and get squirrely when too much rear brake fluid pressure is supplied. For these reasons, a brake proportioning valve is always required in automotive brake systems. These valves are sometimes referred to as a brake metering valve or a brake bias valve. If the valve has other features, like brake light pressure switch or an adjustable rear circuit these are often referred to as a combination valve.

What is the Difference Between Adjustable and Non-adjustable Proportioning Valves?

Proportioning valves are offered in both adjustable and non-adjustable styles. All OEM vehicles use a non-adjustable proportioning valve which is engineered to work properly with the specific front and rear brakes that are on the vehicle, as well as the vehicle weight and tire sizes used. We typically recommend non-adjustable valves only be used when brake systems are very similar to the factory system that the proportioning valve was designed for, such as this disc/drum valve shown here for many common GM cars. High performance brake systems such as those equipped with 4-piston or 6-piston calipers, we always suggest an adjustable proportioning valve. The adjustable valve will allow the driver to increase or decrease the amount of rear braking force so that it can be matched to the front. Once properly adjusted the vehicle will stop safely without losing control or having the rear end step out sideways.

We even offer an adjustable valve with brake light switch function if your street rod pedal assembly is not conducive to a pedal activated switch.

How Do I Adjust an Adjustable Proportioning Valve?

Brake proportioning valve adjustment may seem like a complicated process, but it is quite simple. For all street driven vehicles, the valve will be placed in the rear brake line to reduce the amount of fluid pressure supplied to the rear brakes. The amount of pressure reduction to the rear is adjustable by turning the knob on the proportioning valve. We suggest starting with a low amount of braking to the rear and slowly adjust the valve to increase pressure as you test drive the vehicle. Too much pressure to the rear brakes can cause the driver to lose control, so always be careful and mindful of your surroundings when performing this procedure.

  1. Start by adjusting the proportioning valve to around 50- to 60-percent reduction in output pressure. Many adjustable proportioning valves such as our adjustable proportioning valve will hit a maximum reduction in this range, so often adjusting the valve to the maximum reduction setting is a good place to start. Check the specifications of the valve you are using to find a starting point. Keep in mind that too little rear braking force is safer to start with than too much rear braking, so error on the side of too little to start testing.
  1. Test drive the vehicle at low speeds and in a safe low traffic area. Start by making light stops from low speed (under 30 mph) and progress to heavier stops and elevated speed if needed. If the front brakes are locking up significantly sooner than the rear brakes, the proportioning valve needs to be adjusted to provide more rear brake pressure. If the rear brakes are locking up before the front brakes do and your valve is already adjusted to the maximum rear brake reduction, then you may have a mismatch of front to rear brake design or some other problem within the system (this is not a common problem).
  1. After adjusting the proportioning valve repeat the test drive as before with low-speed light stops and progressing to heavier stops. Continue to adjust the valve to allow more rear braking power until the rear brakes start to lock up at the same point or slightly before the front brakes. Once the rear brakes are stopping at the same point as the front, the valve needs to be adjusted slightly to reduce the rear braking point. The goal is to have the rear brakes contributing as much stopping power as possible but locking up just after the front brakes do.

Updated by Mark Houlahan

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