In the past we have discussed the brake system as well as differences between disc brakes and drum brakes. We have also covered the conversion from drum brakes to disc brakes, but sometimes over-looked in the process is the brake proportioning valve analysis.
Brake proportioning valves are the parts of a vehicle’s brake system that limit the amount of pressure reaching the rear brakes, which helps to prevent rear wheel lock-up, allowing you to stop your vehicle more safely and efficiently.
Brake Proportioning Valve Analysis
Determining when and why to add the brake proportioning valves to the possible causes of the problem being diagnosed.
Cause
Not understanding the proportioning valve can result in the valve being added to the list of possible cases when it cannot cause the diagnosed problem or add it to the list when it should be.
Solution
The proportioning valve is there to prevent rear-wheel lockup during panic braking. Its two main reasons are: 1. most vintage vehicles use drum brakes in the rear, and 2. all vehicles experience weight transfer during a panic stop. Rear drum brakes are hydraulic over mechanical in design.
If applying too much pressure to the wheel cylinders, lockup will occur. All vehicles with rear drum and rear disc brakes will experience weight transfer during a panic stop. When taking off the rear wheels’ weight during a panic stop, the tendency for the rear brakes to lock up will increase. The design of the proportioning valve is there to limit the pressure to the rear brakes during a panic braking situation.
When Do Brake Proportioning Valves Work?
The most crucial point to understand about proportioning valves is when they work – only during panic braking. This means that a vehicle could potentially go its whole life without ever using its proportioning valve. Located between the master cylinder and rear brakes, the conventional proportioning valves are hydraulically activated.
Vehicles with front-to-back split hydraulic systems have one, while diagonal splits systems require two because each rear wheel is on a separate hydraulic circuit. The proportioning valve may be a stand-alone valve, part of the combination valve, or built into the master cylinder.
Deeper Inspection
Whether or not you take the additional steps to inspect or diagnose a vehicle’s hydraulic proportioning valve will depend on the following:
- Could the issue involve the proportioning valve?
- What is the condition and location of the combination valve?
- What type of proportioning valve does the vehicle have?
The only time that the proportioning valve should be on our list of possible causes if your brake complaint involves rear wheel lockup under panic braking. So, ask yourself when does the rear wheel lockup is occurring – light to regular or heavy to panic braking?
If the answer is panic braking, the valve is on our list, but it will not be the only thing on our list. The valve’s design will determine how and when we are going to diagnose it.
Brake Proportioning Valve Cap Screw
The design of some proportioning valves allows two things – more chance for failure and an easy way to diagnose them. If we look at the proportioning valve, we will see the cap screw that holds the piston and spring in place is vented.
The vent hole is a small rubber “mushroom” that prevents moisture from entering the cap screw. If this valve style is mounted low on the vehicle, as in most rear-wheel drive vehicles, it can introduce a high degree of moisture.
This is especially true in areas that use road salts for snow and ice removal. The moisture can corrode the cap screw and allow water to enter where the piston travels in the cap screw.
This causes the cap screw to rust, which bonds the piston and cap screw together.
Typically, after removing the cap screw, the cap screw, spring, and piston are separate pieces. This corrosion prevents the piston from moving when a panic braking situation occurs.
Checking the Proportioning Valves
The same thing that allows the valve to fail will enable us to check it – the cap screw vent hole. First, determine if the valve is a good candidate by its external appearance. If corrosion occurs outside the valve or the cap screw, it is a good candidate. Remove the rubber mushroom from the cap screw.
Next, insert a paper clip to the bottoms out of the piston. Have an assistant start the vehicle and apply heavy pedal pressure to simulate a panic braking situation. The paper clip should push against you and travel about 1/32” to 1/16” as the piston moves. If the paper clip does not move, the piston has seized in the cap screw, and the valve will need replacing. This test will work on any proportioning valve vented through the cap screw and should be part of your standard inspection if the valve’s exterior condition merits it.
Checking the Proportioning Valves (Not Vented)
Many proportioning valves are not vented through the cap screw. There is not a quick check for these types of valves. These valves should only be checked if the customer’s complaint indicates a possible proportioning valve problem. Most shops do not have pressure gauges, so diagnosing the issue will have to be eliminated. You should check all other possible causes before condemning the proportioning valve.
Pressure gauges are the best way to diagnose a failed proportioning valve. There will be a gage installed into the front and rear hydraulic circuits. Start the vehicle and apply the brake pedal with heavy pedal pressure to duplicate a panic braking situation. The front and rear pressures should be different. Most manufacturers do not publish these pressures. The rear brakes usually are “shut” down between 500psi and 800psi, while the fronts can climb to well over 1500psi.
What you are looking for is whether the rear pressure has any limitation. If the rear pressure is over 1000psi and is close to or equal to the front pressure, then the valve is not working and will have to be replaced to correct the problem.
Proportioning Valves Rules of Thumb
Using the definition of the proportioning valve’s function and our understanding of system operation, we can use the following rules of thumb:
- On the front to rear split hydraulic systems, a failed proportioning valve should cause both rear wheels to lock up under panic braking – unless one rear brake has a condition that will prevent this (i.e., severely out of adjustment or seized wheel cylinder).
- On diagonal split hydraulic systems (FWD), it will be infrequent for both proportioning valves to fail simultaneously. If both rear wheels are locking under heavy to panic, look at mechanical causes before condemning the proportioning valves.
Finally
Whether the car is equipped with front disc and rear drums, or has four-wheel discs, hydraulic pressure should be reduced to the rear brakes for several reasons. The single most important reason is because even under moderate braking, natural weight transfer reduces weight on the rear tires and moves the weight to the front. This unloads the rear tires, which makes it much easier to lock up the rear brakes in hard braking. Once the rear brakes are locked, the tires skid, which immediately causes loss of control.
A properly installed and adjusted brake proportioning valve can dramatically improve stopping distance, not to mention bolster your confidence. When it comes to stopping safely, nobody wants to be slip sliding away and that is why we have provided you with the basics in brake proportioning valve analysis.
