Easy Way to Tell if Proportioning Valve is Working
Problem
Cause
Solution
The proportioning valve is there to prevent rear-wheel lockup during panic braking. Its two main reasons are 1. most 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 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 customer's complaint 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 the customer's complaint involves rear wheel lockup under panic braking. We must ask the customer when 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 won't be the only thing on our list (See pg 104 for more information on rear wheel lockup). The valve's design will determine how and when we are going to diagnose it.
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 in Figure 49.1, 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 into where the piston travels in the cap screw (See Figure 49.2). This causes the cap screw to rust, which bonds the piston and cap screw together (See Figure 49.3).
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 (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 See Figures 49.5, 49.6, and 49.7).
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.
More Tips and Tricks
Source: https://www.brakebleeder.com/proportioning-valve-diagnosis/
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