How Master Cylinders and Combination
Valves Work by Karim
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 edition of HowStuffWorks,
we will learn how the master cylinder and combination valve
work. If you haven't read about basic car braking concepts in
Work, be sure to check it out.
The Master Cylinder Here is where you'll
find the master cylinder:
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.
The master cylinder, reservoir and
As you'll see here, there are two pistons and two springs
inside the cylinder.
Diagram of master
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
The valve does the job of three separate devices:
The metering valve
The pressure differential switch
The proportioning valve
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.
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 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.