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ABOUT BRAKES
MASTER CYLINDER
The master cylinder displaces hydraulic pressure
to the rest of the brake system. It holds THE most important fluid in your car,
the brake fluid. It actually controls two seperate subsystems which are jointly
activated by the brake pedal. This is done so that in case a major leak occurs
in one system, the other will still function. The two systems may be supplied by
seperate fluid reservoirs, or they may be supplied by a common reservoir. Some
brake subsystems are divided front/rear and some are diagonally separated. When
you press the brake pedal, a push rod connected to the pedal moves the "primary
piston" forward inside the master cylinder. The primary piston activates one of
the two subsystems. The hydraulic pressure created, and the force of the primary
piston spring, moves the secondary piston forward. When the forward movement of
the pistons causes their primary cups to cover the bypass holes, hydraulic
pressure builds up and is transmitted to the wheel cylinders. When the brake
pedal retracts, the pistons allow fluid from the reservoir(s) to refill the
chamber if needed.
Electronic sensors within the master cylinder are
used to monitor the level of the fluid in the reservoirs, and to alert the
driver if a pressure imbalance develops between the two systems. If the brake
light comes on, the fluid level in the reservoir(s) should be checked. If the
level is low, more fluid should be added, and the leak should be found and
repaired as soon as possible. BE SURE TO USE THE RIGHT BRAKE FLUID FOR YOUR
VEHICLE. Use of improper brake fluid can "contaminate the system". If this
occurs, ALL of the seals in the brake system will need replacement, and that is
usually a VERY expensive operation.
BRAKE WARNING SYSTEM
The brake warning system has been required
standard equipment since 1970, and is connected to the master cylinder. It
monitors differences in pressure in the brake lines of the two hydraulic
sub-systems, and alerts the driver with a light if an imbalance occurs. When you
turn the key to the Ignition position, the brake warning light on the dash comes
on during a "self-test". You should not drive a car if the warning light does
not come on during the startup self test.
The brake system is divided into two sub-systems
to increase safety. A pressure differential switch, connected to the warning
light, is positioned between the two. If a major leak occurs, and therefore
pressure in one of the lines is sharply reduced, pressure from the other side
forces a piston to move, activating the pressure differential switch and turns
on the dashboard warning light.
There are two types of pressure differential
switches; mechanical or hydraulic. Mechanical switches are activated by
excessive brake travel. Hydraulic switches are activated by a difference in
pressure between the front and rear system. When pressure in one of the lines is
sharply reduced, pressure from the other side forces a piston to move. A plunger
pin then drops into a groove in the piston, activating a switch that turns on a
dashboard warning light.
The brake warning light is also connected to the
brake fluid level sensors in the master cylinder reservoir(s). If the brake
warning light comes on, the fluid level should be checked. If the level is low,
more fluid should be added, and the leak should be found and repaired as soon as
possible. BE SURE TO USE THE RIGHT FLUID. NEVER IGNORE THE BRAKE WARNING LAMP,
AND ALWAYS NOTE WETHER IT WORKS DURING THE STARTING SELF-TEST.
POWER BRAKES
Power brakes (also called "power assisted"
brakes) are designed to use the power of the engine and/or battery to enhance
braking power. The four most common types of power brakes are: vacuum suspended;
air suspended; hydraulic booster, and electro-hydraulic booster. Most cars use
vacuum suspended units (vacuum boosters), which employ a vacuum-powered booster
device to provide added thrust to the foot pressure applied.
In a vacuum booster type system, pressure on the
brake pedal pushes forward a pushrod connected to the pistons within the master
cylinder. At the same time, the pushrod opens the vacuum-control valve so that
it closes the vacuum port and seals off the forward half of the booster unit.
The engine vacuum line then creates a low-pressure vacuum chamber. Atmospheric
pressure in the control chamber then pushes against the diaphragm. The pressure
on the diaphragm forces it forward, supplying pressure on the master cylinder
pistons.
Hydraulic booster systems usually tap into the
power steering pump's pressure, and use this power to augment pressure to the
master cylinder. Electro-hydraulic booster systems use an electric motor to
pressurize a hydraulic system which augments pressure to the master cylinder.
This allows the vehicle to have power assisted brakes even if the engine quits.
You may wish to compare the difference between
power and non-assisted braking in a safe area; while driving slowly, turn the
ignition key off (don't turn it into the locked position, because the steering
wheel will lock, which is highly unsafe.) As the car coasts along, press the
brakes hard. The force of your foot is now the only thing stopping the car. The
safe driver is always ready to apply the total force needed to stop their
vehicle, even if the engine quits (thereby removing the power assist).
FILLER CAP
The cap on the brake fluid reservoir has a hole
for air, or is vented, to allow the fluid to expand and contract without
creating a vacuum or causing pressure. A rubber diaphragm goes up and down with
the fluid level's pressure, and keeps out any dust or moisture. If the cap's
seal becomes distorted, it usually indicates a brake fluid contamination
problem.
VACUUM FROM THE ENGINE
Engine intake manifold vacuum is used for
augmenting the foot's braking power in vacuum assisted power brakes. This vacuum
is created by the pistons as they draw downward, sucking air into the cylinders.
When you push the brake pedal down, the vacuum control valve lets the engine
draw a vacuum in the front section of the booster unit. The atmospheric pressure
on the other side of the diaphragm provides significant additional braking
force.
BRAKE FLUID
Brake fluid is a special liquid for use in
hydraulic brake systems, which must meet highly exact performance
specifications. It is designed to be impervious to wide temperature changes and
to not suffer any significant changes in important physical characteristics such
as compressibility over the operating temperature range. The fluid is designed
to not boil, even when exposed to the extreme temperatures of the brakes.
Different types of brake fluid are used in
different systems, and should NEVER be mixed. Most cars use "DOT 3" or "DOT 4"
brake fluid. Some newer cars use silicone brake fluids. These should NEVER be
mixed together, because the seals in each car are designed to work with only
their specific fluid types. For example, the mixing of "Silicone" brake fluid
and conventional glycol based DOT 3 or DOT 4 fluids should be avoided, as the
two fluid types are not miscible (they will not mix together). DOT 3 brake
fluids and DOT 4 brake fluids can be mixed.
One of the WORST things that can happen to your
car is if the brake fluid becomes contaminated, because the seals are designed
to work with only pure brake fluid. "System contamination" means that all of the
piston seals and hoses are deteriorating, and therefore must be replaced, a
MAJOR expense. So, be VERY careful what you put in the master cylinder
reservoir!
It should be noted that brake fluid is highly
corrosive to paint, and care should be used not to get it on your car's finish.
The brake fluid in your car should be changed
every (See Owners Manual) to prevent corrosion of the braking system
components.
We all know that pushing down on the brake pedal
slows a car to a stop. But how does this happen? How does your car transmit the
force from your leg to its wheels? How does it multiply the force so that it is
enough to stop something as big as a car?
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