How Master Cylinders and Combination Valves Work
To increase safety, most modern car brake systems are broken into two circuits, with two wheels on each circuit. If a fluid leak occurs in one circuit, only two of the wheels will lose their brakes and your car will still be able to stop when you press the brake pedal.
The master cylinder supplies pressure to both circuits of the car. It is a remarkable device that uses two pistons in the same cylinder in a way that makes the cylinder relatively failsafe. The combination valve warns the driver if there is a problem with the brake system, and also does a few more things to make your car safer to drive.
In this article, we will learn how the master cylinder and combination valve work. If you haven't read about basic car braking concepts in How Brakes Work, be sure to check it out.
The Master Cylinder
Here is where you'll
find the master cylinder:
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In the figure below, the plastic tank you see is the brake-fluid reservoir, the master cylinder's brake-fluid source. The electrical connection is a sensor that triggers a warning light when the brake fluid gets low.
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As you'll see here, there are two pistons and two springs inside the cylinder.
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The Master Cylinder in
Action
When you press the brake pedal, it pushes on
the primary piston through a linkage. Pressure
builds in the cylinder and lines as the brake pedal is
depressed further. The pressure between the primary and
secondary piston forces the secondary piston to
compress the fluid in its circuit. If the brakes are operating
properly, the pressure will be the same in both circuits.
If there is a leak in one of the circuits, that circuit will not be able to maintain pressure. Here you can see what happens when one of the circuits develops a leak.
Master cylinder with leak
When the first circuit leaks, the pressure between the primary and secondary cylinders is lost. This causes the primary cylinder to contact the secondary cylinder. Now the master cylinder behaves as if it has only one piston. The second circuit will function normally, but you can see from the animation that the driver will have to press the pedal further to activate it. Since only two wheels have pressure, the braking power will be severely reduced.
The Combination Valve
You will find a
combination valve on most cars with front
disc
brakes and rear drum
brakes.
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The valve does the job of three separate devices:
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Metering Valve
The
metering valve section of the combination valve is required on
cars that have disc brakes on the front wheels and drum brakes
on the rear wheels. If you have read How Disc
Brakes Works and How Drum
Brakes Work, you know that the disc brake pad is normally
in contact with the disc, while the drum brake shoes are
normally pulled away from the drum. Because of this, the disc
brakes are in a position to engage before the drum brakes when
you push the brake pedal down.
The metering valve compensates for this, making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached. The threshold pressure is low compared to the maximum pressure in the braking system, so the drum brakes just barely engage before the disc brakes kick in.
Having the rear brakes engage before the front brakes provides a lot more stability during braking. Applying the rear brakes first helps keep the car in a straight line, much like the rudder helps a plane fly in straight line.
Pressure Differential
Switch
The pressure differential valve is the device
that alerts you if you have a leak in one of your brake
circuits. The valve contains a specially shaped piston in the
middle of a cylinder. Each side of the piston is exposed to
the pressure in one of the two brake circuits. As long as the
pressure in both circuits is the same, the piston will stay
centered in its cylinder. But if one side develops a leak, the
pressure will drop in that circuit, forcing the piston
off-center. This closes a switch, which turns on a light in
the instrument panel of the car. The wires for this switch are
visible in the picture above.
Proportioning
Valve
The proportioning valve reduces the pressure
to the rear brakes. Regardless of what type of brakes a car
has, the rear brakes require less force than the front brakes.
The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.
If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.
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