Brake boosters (vacuum assist) — source of assist force In a typical vacuum brake booster, what provides the assist that reduces the pedal effort required by the driver?

Introduction / Context:
Most conventional passenger cars use a vacuum booster to assist braking. The component multiplies pedal force so that the master cylinder can generate adequate hydraulic pressure. Understanding the energy source behind this assist is fundamental to brake diagnostics and safety.

Given Data / Assumptions:

• Spark-ignition engine producing manifold vacuum during part-throttle operation.
• Vacuum reservoir and check valve installed.
• Not an electro-hydraulic or full electric brake system.

Concept / Approach:
The booster has two chambers separated by a diaphragm. With the pedal released, both chambers are at manifold vacuum. When the pedal is pressed, a control valve admits atmospheric air to the rear chamber while the front remains under vacuum. The pressure difference across the diaphragm applies an assist force to the pushrod, reducing required pedal effort.

Step-by-Step Solution:
Engine creates vacuum in the intake manifold.Check valve stores vacuum and prevents loss when throttle is opened.Pedal actuation opens an atmospheric port on one side, producing a pressure differential that assists the pushrod.

Verification / Alternative check:
With the engine off, repeatedly pressing the pedal removes stored vacuum and pedal effort increases, demonstrating reliance on atmospheric-to-vacuum differential.

Why Other Options Are Wrong:
Exhaust pressure is not connected to the booster. Electric motors are used in some brake-by-wire systems, but not in a vacuum booster. Hydraulic pump pressure powers hydro-boost systems, not vacuum boosters.

Common Pitfalls:
Misdiagnosing a hard pedal as a hydraulic fault when it may be a vacuum leak or failed check valve. Confusing vacuum boosters with hydro-boost units used on diesel engines.

Final Answer:
pressure difference between atmospheric air and intake manifold vacuum