Brake Valve Problem:
Determining whether to add the vehicle’s brake valve(s) to the list of possible causes.
The Cause of Difficulty in Brake Valves:
Brake valves represent the least understood of all the base brake components. Not knowing what the various brakes do, when they do, and how they do it can lead to misdiagnosis.
Brake Valve Solution:
Brake valves are the least understood of all conventional brake components. By understanding what brake valves a vehicle has is essential to the accurate diagnosis of specific brake problems. Because of this, a technician should determine a vehicle’s brake valves by knowing the hydraulic system the car has and looking at the valve itself.
Determining a vehicle’s hydraulic system is very easy on most vehicles. For example, FWD vehicles have diagonal split hydraulic systems (See Figure 47.1). As a result, a diagonal split hydraulic system connects each front wheel with the opposite rear wheel. Likewise, the majority of RWD vehicles have front to rear split hydraulic systems (See Figure 47.2). In a front-to-rear split hydraulic system, the front section is connected hydraulically to one half of the tandem master cylinder while the rear section connects to the other half.
There are some exceptions where RWD vehicles have diagonal split hydraulic systems. Fords and Lincolns are the most common of these exceptions. The easiest way to determine the system is to look at the rear brake lines. For example, if the brake lines from each rear wheel come together into a central line, the vehicle has a front to the rear split hydraulic system. If each rear line continues to the front of the vehicle, the system is a diagonal split hydraulic system.
Looking at Brake Valve
The next step in identifying the type of brake valve(s) a vehicle has involves looking at the brake valve(s). For example, the majority of late-model cars use combination valves. A combination valve is a valve that “combines” more than one valve into a standard housing. Similarly to above, determining the combination valve involves a simple inspection process.
Figure 47.3 shows two typical RWD combination valves. These valves house three separate valves in ordinary housing. The first and easiest valve to identify in this type of valve is the pressure differential switch. The pressure differential switch is in the center of the combination valve (Labeled as “B” in Figure 47.3). The plastic switch identifies it. Any conventional brake valve with this type of switch is a pressure differential switch. For example, the pressure differential switch will complete ground to the red brake warning light in a hydraulic failure.
The next valve to determine will be the valve between the master cylinder and rear brakes. To do this, trace the line from the rear brakes to the combination valve. As you find the end of the valve, you’ll see that it houses the proportioning valve. The proportioning valve prevents rear wheel lockup during panic braking. A hydraulic activated valve starts limiting the pressure to the rear brakes at a certain point called the “knee point.” The proportioning valves have the “C” label in Figures 47.3.
Metering Brake Valve
Finally, the last valve to be identified in a typical combination valve is the metering valve (Labeled as “A” in Figure 47.3). You can locate the metering valve between the master cylinder and front brakes. The majority of RWD vehicles have disc brakes on the front, while drum brakes on the rear have a metering valve. As a result, you should look for the presence of a stem. For example, the stem may protrude from the rubber cap located at the end of the valve, or the rubber cap may cover it up.
The metering valve’s function is to hold off the front brake application until the rear brakes overcome the return springs. The valve does this to allow all brakes to apply at the same time. The combination valve pictured in Figure 47.4 doesn’t have a metering valve. The valve only serves as a junction block for the front brake lines and a proportioning valve.
The next category of brake valves to look at are those combination valves used on FWD vehicles. The majority of brake valves used on FWD vehicles will be dual proportioning valves combined into a standard housing, as shown in Figure 47.5. Each rear brake on a diagonal split hydraulic system is on a separate circuit. Because of the separation, FWD vehicles have two proportioning valves.
Combining the two proportioning valves into standard housing is the most frequently used method. Older FWD vehicles may also include the pressure differential switch in the combination valve, as shown in Figure 47.6. Newer FWD vehicles use a fluid level switch in the master cylinder reservoir instead of the pressure differential switch.
FWD Combination Brake Valve
An important point to understand about FWD combination valves is that although the lines from the front wheels connect to the valve, fluid flow is unrestricted through the valve. The valve serves only to accomplish the diagonal split “plumbing” but does not act on the fluid flow to the front brakes. FWD vehicles don’t have metering valves or any valve that would limit front braking. The reason for this is due the front brakes do most of the work.
While the majority of FWD vehicles combine the proportioning valves into a familiar housing, others do not. For example, Figures 47.7 & 47.8 show two alternate methods of equipping an FWD vehicle with two proportioning valves. Figure 47.7 shows screw-in proportioning valves used on some four outlet master cylinders, while Figure 47.8 shows side-by-side proportioning valves used on some other FWD vehicles. Some FWD vehicles build the proportioning valves into the master cylinder, as shown in Figure 47.9.
Height Sensitive Brake Valve
The proportioning valves pictured in the previous Figures are hydraulic activated valves. Likewise, they rely on system pressure to know when to operate. However, there is another category of proportioning valve that depends on vehicle ride height to function. Typically, we call these valves height-sensitive control valves, load sensing proportioning valves, or “smart” valves (See page 88 for more detail). The identifying characteristic is the presence of linkage between the valve and the vehicle’s suspension.
For example, there are two typical height-sensitive control valves shown in Figure 47.10. The top valve is from a car equipped with a front to rear split hydraulic system. The bottom valve is from a vehicle equipped with a diagonal split hydraulic system. This valve is a type of combination valve because it houses two separate valves in the same housing.
Conventional brake valving may be attached or incorporated into the ABS. The ABS modulator shown in Figure 47.11 has the combination valve attached directly to it. They appear as one unit but are two separate parts. Some ABS systems use screw-in proportioning valves similar to those used in some master cylinders.
Some newer vehicles will not have any mechanical brake valves. Because advances in computer technology have allowed the ABS computer to take over the job of the proportioning valve. These systems let the rear brakes operate up to the point of wheel lock-up and then use micro-pulses to control the pressure to avoid the lock-up. The mico-pulses do not produce the traditional ratcheting pedal drop as experienced during a usual ABS stop. They have tiny and very rapid changes in pressure, which in many cases is unnoticeable in the pedal.
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