1. Introduction
With the rapid development of automobile speed, hydraulic braking systems are required to provide extremely high hydraulic pressure—often in the range of 10–20 MPa—to generate braking torque that matches vehicle speed. It is almost impossible to achieve this level of braking force solely by human pedal effort. This limitation is even more pronounced in disc brake systems, which lack any self-servo effect and thus rely entirely on increased hydraulic pressure.
To address this issue, modern hydraulic brake systems are widely equipped with a Vacuum Brake Booster (Vacuum Hydraulic Brake Transmission System). By utilizing vacuum created in the engine intake manifold, the booster reduces pedal effort, enhances braking force at the wheels, and ensures light and safe operation.
Vacuum-assisted brake systems are generally classified into two types:
- Vacuum Hydraulic Booster (Vacuum Pressure Booster): Mounted after the brake master cylinder, this unit amplifies the hydraulic pressure output by the master cylinder before transmitting it to the wheel cylinders.
- Vacuum Brake Servo (Vacuum Servo Booster): Mounted between the brake pedal and the master cylinder, it directly assists the pedal force applied to the master cylinder piston.
2. Vacuum Hydraulic Booster System
2.1 Composition and Principle
The vacuum hydraulic booster system consists of:
- Power Supply Device: engine intake manifold, vacuum one-way valve, and vacuum reservoir.
- Control Device: control valve.
- Transmission Device: vacuum servo chamber, auxiliary cylinder, and safety cylinder.
Working principle:
- When the engine runs, vacuum from the intake manifold is stored in the reservoir via the one-way valve, serving as the energy source for brake boosting.
- When the brake pedal is pressed:
- The brake master cylinder sends fluid into the auxiliary cylinder, transmitting part of it to the front and rear wheel cylinders.
- At the same time, hydraulic pressure acts as a control signal on the control valve.
- The servo chamber produces additional thrust, combining with hydraulic pressure to push the auxiliary cylinder piston, thereby outputting a much higher brake pressure than the master cylinder alone.
- The safety cylinder ensures that if one brake circuit fails, the other circuit remains functional by automatically sealing off the damaged side.
2.2 Structure of the Vacuum Hydraulic Booster
The booster mainly includes:
- Auxiliary Cylinder: Amplifies low-pressure fluid into high-pressure. Its piston, equipped with a ball valve, separates left and right chambers.
- Control Valve: Composed of vacuum and air valves, responsible for switching the servo chamber between vacuum and atmosphere.
- Vacuum Servo Chamber: Divided into front and rear chambers by a diaphragm. By exploiting the pressure difference between vacuum and atmospheric pressure, it generates thrust to drive the push rod.
2.3 Working Principle
- Non-braking (Rest Condition): Four chambers (A, B, C, D) are interconnected, vacuum is equalized, and the auxiliary cylinder chambers are open, keeping the system balanced.
- Braking Process:
- Master cylinder pressure enters the auxiliary cylinder.
- Control valve piston closes the vacuum valve and opens the air valve.
- Atmospheric pressure enters chamber D, while chamber C remains under vacuum, creating a pressure difference.
- The diaphragm pushes the rod, sealing the auxiliary cylinder, and the left chamber pressure rises sharply, transmitting higher hydraulic pressure to wheel cylinders.
- Holding (Maintaining Braking): Both air and vacuum valves are closed, forming a double-valve closed condition, which keeps the pressure constant.
- Release: When the pedal is released, control pressure drops, the vacuum valve opens, chambers re-connect, and the diaphragm, piston, and push rod return under spring force, releasing the brake.
2.4 Testing and Inspection
Performance of the vacuum booster directly affects braking efficiency. Inspections include:
- Simple Tests:
- Pedal height difference test with engine on/off.
- Control valve check for leakage or malfunction.
- Diaphragm stroke measurement through inspection ports.
- Instrument Tests:
- Air tightness: Vacuum drop within 15s should not exceed 3.4 kPa.
- Oil tightness: Pressure drop within 10s should not exceed 1 kPa.
- One-way valve test: Vacuum holding time should be ≥15 seconds.
3. Vacuum Brake Servo System
3.1 System Composition
Taking the Audi 100 passenger car as an example, a typical dual-circuit vacuum servo brake system includes:
- Tandem dual-chamber master cylinder.
- Vacuum servo chamber + control valve (collectively known as the vacuum servo unit).
- Vacuum one-way valve.
- Proportioning valve and wheel cylinders (front and rear).
3.2 Structure of the Vacuum Servo Unit
A common design is the single-diaphragm vacuum servo booster (used in vehicles like Santana):
- Formed by front and rear housings clamping a diaphragm and valve body.
- The front chamber connects to the intake manifold or vacuum tank.
- The rear chamber houses the air valve and vacuum valve, linked to the brake pedal push rod.
3.3 Working Principle
- Non-braking: The vacuum valve is open, air valve closed, both chambers A and B maintain vacuum, and pedal effort is light.
- Braking: The pedal push rod closes the vacuum valve and opens the air valve, allowing atmospheric pressure into chamber B. The pressure difference between chamber A (vacuum) and chamber B (atmosphere) drives the diaphragm forward, amplifying the hydraulic output.
- Holding: Both valves close, creating balance. Pressure in the system remains constant, maintaining braking force.
- Release: When the pedal is released, the vacuum valve reopens, chambers reconnect to vacuum, and the diaphragm and push rod return to rest under spring action.
- Failure Condition: If the booster fails, the pedal directly drives the master cylinder. Braking is still possible but requires much higher pedal force.
3.4 Testing and Validation
- On-vehicle test: Pedal height changes with engine running vs. stopped indicate booster condition.
- Vacuum holding test: After disconnecting the vacuum source, the system should maintain vacuum for at least 15 seconds without significant drop.
- One-way valve test: Checked with a hand vacuum pump to ensure it holds vacuum properly.
4. Conclusion
The Vacuum Hydraulic Brake Transmission System plays a crucial role in modern vehicles:
- Vacuum Hydraulic Booster: Amplifies hydraulic pressure significantly through the servo chamber and auxiliary cylinder, widely used in medium trucks and commercial vehicles.
- Vacuum Brake Servo: Directly assists the master cylinder, compact and efficient, commonly applied in passenger cars and light vehicles.
Key advantages:
- Reduces driver pedal effort.
- Improves braking safety and stability.
- Provides redundancy in case of partial system failure.
With the ongoing trend of vehicle electrification and automation, vacuum-based boosters are gradually being replaced by electronic brake boosters (E-Booster). However, the fundamental principle of using differential pressure for brake assistance, and its strict safety requirements, continue to serve as the foundation of modern automotive braking systems.
Learn More About Brake Components
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What Is a Brake Master Cylinder? (Comprehensive Guide) — Detailed explanation of its role in converting pedal force into hydraulic pressure.
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What is a Brake Wheel Cylinder? Function, Symptoms, Repair & Cost — Understanding the key component of drum brake systems.
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Clutch Master Cylinder Full Guide: Definition, Symptoms, Repair & Cost — Everything you need to know about clutch hydraulics.
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What Is a Clutch Slave Cylinder? — How it works to disengage and engage clutch systems smoothly.
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What Is a Brake Proportioning Valve? — Why it’s essential for balancing braking force across axles.
Explore More
Looking for more related components? Explore our product categories to find the right fit for your vehicles and get customized solutions:
- Brake Master Cylinder — The “heart” of the hydraulic system, supporting multiple vehicle platforms with customizable options.
- Brake Wheel Cylinder — Directly drives brake shoes or calipers, ensuring stable and reliable braking performance.
- Clutch Master Cylinder — Precision hydraulic source for smooth and effortless clutch operation.
- Clutch Slave Cylinder — Robust design for quick and accurate clutch disengagement.
- Brake Proportioning Valve — Smartly balances front and rear braking forces to enhance stability.
- Brake Accessories — One-stop supply of reservoirs, seals, return springs, and more.
👉 Click on any category above to view detailed specifications and request a tailored solution for your vehicle platform.
