BrakesBrake Assist System Description and Operation
Brake Assist System Description and Operation
System Component Description
The brake assist system consists of the following:
| Brake Pedal: Receives, multiplies and
transfers brake system input force from driver. |
| Brake Pedal Pushrod: Transfers multiplied
input force received from brake pedal to brake booster. |
| Vacuum Brake Booster: Uses source vacuum to
decrease effort required by driver when applying brake system input
force. At rest, source vacuum is applied to both sides of the
vacuum diaphragm for single boosters, or to both sides of each of
the diaphragms for tandem boosters. Return springs maintain the
booster in a rest position. When brake system input force is
applied, vacuum to the rear of the diaphragm(s) is cut off, and air
at atmospheric pressure is admitted in its place. This provides a
decrease in brake pedal effort required. When input force is
removed, vacuum again replaces atmospheric pressure within the
booster. |
| Vacuum Source: Supplies force used by vacuum
brake booster to decrease brake pedal effort. The primary source
for vacuum is typically the vehicle's internal combustion engine.
Some vehicles may use a vacuum pump to ensure and adequate supply
of vacuum under various operating conditions, such as during cold
start-up, heavy throttle, and high altitude. |
| Vacuum Pump Relay: Vehicles equipped with a
vacuum pump will typically be equipped with a relay used to provide
voltage to the vacuum pump motor, when commanded by the powertrain
controller. |
| Vacuum Monitoring System: Provides a feedback
voltage signal relative to the amount of vacuum available for the
vacuum power brake booster. This typically consists of a vacuum
sensor mounted either into the power vacuum brake booster,
integrated with the vacuum check valve, or in-line within the
vacuum hose. The sensor feedback is typically monitored by the
powertrain control module(s). |
| Vacuum Source Control: If equipped with a
system to monitor available levels of vacuum for the brake assist
system, the powertrain control module(s) typically perform the
primary monitoring function. The powertrain controller will
typically monitor the feedback signal of a vacuum sensor, as it
relates to available vacuum at the vacuum power brake booster. The
powertrain controller uses the information to make sure available
vacuum remains above predetermined levels. If additional vacuum is
required, the powertrain controller may reactivate engine
cylinders, if equipped, or command a vacuum pump ON, if equipped,
and/or may alter the operation of various other engine controls and
devices, as necessary. |
| Vacuum Source Delivery System: Enables
delivery and retention of source vacuum for vacuum brake booster.
This typically consists of a vacuum check valve and a vacuum hose
or pipe. If a vacuum pump is used, additional check valves may be
used. |
System Operation
Brake system input force from the driver is multiplied by the
brake pedal and transferred by the pedal pushrod to the hydraulic
brake master cylinder. Effort required to apply the brake system is
reduced by the operation of the vacuum brake booster.
When brake system force is applied, vacuum to the rear of the
diaphragm(s) is cut off, and air at atmospheric pressure is
admitted in its place. The vacuum acts to pull the diaphragm(s)
forward, while the atmospheric pressure acts to push from behind.
As a result, the amount of effort required by the driver is
reduced. When input force is removed, vacuum again replaces
atmospheric pressure within the vacuum power brake booster to help
return the booster to rest. Return springs maintain the system in a
rest position.
If the vehicle is equipped with a vacuum monitoring system, the
powertrain control module(s) typically will alter powertrain
functions and/or command the vacuum pump ON, if equipped, to
provide for the vacuum needs of the brake assist system, as
operating conditions warrant.
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07 Brakes
