Modern cars have brakes on all four wheels, operated by a hydraulic system. The breaks may be a disc type or drum type.
The front brakes play a greater part
in stopping the car than the rear ones,
because braking throws the car weight
forward on to the front wheels.
Many cars therefore have disc
brakes, which are generally more effi-
cient, at the front and drum brakes at
All-disc braking systems are used on
some expensive or high-performance
cars, and all-drum systems on some
older or smaller cars.
A hydraulic brake circuit has fluid-
filled master and slave cylinders con-
nected by pipes.
When you push the brake pedal it
depresses a piston in the master cylin-
der, forcing fluid along the pipe.
The fluid travels to slave cylinders
and fills them, forcing pistons out to apply the breaks.
Fluid pressure distributes itself evenly around the system. The combined surface pushing area of all the slave pistons is much greater
than that of the piston in the master
Consequently, the master piston has
to travel several inches to move the
slave pistons the fraction of an inch it
takes to apply the brakes.
This arrangement allows great force
to be exerted by the brakes, in the
same way that a long-handled lever
can easily lift a heavy object a short
Most modern cars are fitted with
twin hydraulic circuits, with two mas-
ter cylinders in tandem, in case one
Sometimes one circuit works the
front brakes and one the rear brakes,
or each circuit works both front brakes
and one of the rear brakes; or one cir.
cuit works all four brakes and the
other the front ones only.
Under heavy braking , so much weight may come off the rear wheels that they lock, possibly causing the car to skid. For this reason, the rear brakes are deliberately made less powerful than the front.
Most cars now also have a loa
sensitive pressure-limiting valve. It
closes when heavy braking raises
hydraulic pressure to a level that
might cause the rear brakes to lock,
and prevents any further movement
of fluid to them.
Advanced cars may even have com-
plex anti-lock systems that sens in
various ways how the car is decelerat-
ing and whether any wheels are
Such systems apply and release the
brakes in rapid succession to stop
Many cars also have power assistance
to reduce the effort needed to apply
Usually the source of power is the pressure difference between the partial vacuum in the inlet manifold and the outside air.
The servo unit that’s provides the assistance has a pipe connection to the inlet manifold.
A direct-acting servo is fitted be-
tween the brake pedal and the master
cylinder. The brake pedal pushes a rod
that in turn pushes the master
But the brake pedal also works on a
set of air valves, and there is a large
rubber diaphragm connected to the
When the brakes are off, both sides of
the diaphragm are exposed to the vac-
uum from the manifold
Pressing the brake pedal closes the
valve linking the rear side of the dia
phragm to the manifold, and opens
a valve that lets in air from outside.
The higher pressure of the outside
air forces the diaphragm forward to
push on the master-cylinder piston,
and thereby assists the braking effort.
If the pedal is then held, and pressed no further, the air valve admits no more air from the outside, so the pressure on the brakes remains the same.When the pedal is released, the space behind the diaphragm is reopened to the manifold, so the pressure drops and the diaphragm falls back.
space behind the diaphragm is
If the vacuum fails because the
engine stops, for example the brakes
still work because there is a normal
mechanical link between the pedal
and the master cylinder. But much
more force must be exerted on the
brake pedal to apply them.
Some cars have an indirect-acting
servo fitted in the hydraulic lines be-
tween the master cylinder and the
brakes. Such a unit can be mounted
anywhere in the engine compartment
instead of having to be directly in
front of the pedal.
It, too, relies on manifold vacuum to
provide the boost. Pressing the brake
pedal causes hydraulic pressure build
up from the master cylinder, a valve opens and that triggers the vacuum servo.