Master Cylinder
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 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 sensor there are two pistons
and two springs inside the cylinder. 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 can leak.
When the first circuit
leaks, the pressure between the primary and secondary cylinders is lost.
This causes the primary master 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.
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