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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Air Brakes: Systems, Service, and Compliance › Lesson 10
TRN-DIES20-03 · Lesson 10 · 14 clock hours · demo

Air-Over-Hydraulic Brake Systems

Objective. Students will describe the components and operation of an air-over-hydraulic (AOH) brake system and identify how air pressure converts to hydraulic line pressure through the master cylinder booster.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

What Is an Air-Over-Hydraulic Brake System?

An air-over-hydraulic (AOH) brake system combines two technologies: compressed air from the vehicle's air supply provides the force, but the actual clamping at each wheel is done by hydraulic fluid under pressure. This hybrid design is common on medium-duty trucks, school buses, and some specialty vehicles that are too heavy for a pure hydraulic system but do not require the full air-brake foundation system used on Class 8 trucks.

The key advantage of AOH is that it uses the power of compressed air—typically 90–120 psi in the reservoir—to multiply force through a small hydraulic master cylinder. The driver applies moderate pedal effort, the air system does most of the work, and hydraulic fluid transmits that amplified force cleanly to the wheel cylinders or calipers.

Major Components

Air supply side

AOH booster/master cylinder assembly

This is the heart of the system. It contains:

Hydraulic side

How Air Pressure Becomes Hydraulic Pressure

Pressure multiplication depends on the ratio of the air piston area to the hydraulic piston area. Because the air piston is large and the hydraulic piston bore is small, a modest air pressure produces a much higher hydraulic pressure. This is Pascal's Law applied in a practical device. A calculation example is given in the Worked Examples section.

Tracing the System

Start at the foot valve. When the driver presses the pedal, metered air travels through the supply line to the booster's air inlet. The control valve inside the booster opens, allowing that air to act on the air piston. The air piston pushes the hydraulic piston forward, pressurizing brake fluid in the master cylinder bore. That fluid pressure travels through steel or nylon-lined hydraulic tubing to each wheel cylinder or caliper, applying the brakes. When the pedal is released, the control valve exhausts air to atmosphere, spring force returns the pistons, and brake fluid returns to the reservoir.

Bleeding the Hydraulic Side

Bleeding AOH hydraulic lines removes air bubbles that cause a spongy pedal. The procedure is similar to bleeding a conventional hydraulic system, but the pressure source is the booster, not a hand pump or helper on the pedal.

AOH-specific considerations:

Safety Rules for This Lesson

PPE is required at all times during hands-on work. Wear safety goggles and chemical-resistant gloves when handling brake fluid—it is harmful to eyes and skin and will damage paint.

Hydraulic pressure hazard: AOH systems can generate hydraulic line pressures in the hundreds to over a thousand psi. Never crack a hydraulic fitting while the air system is pressurized and the booster is active. Depressurize the air system and confirm zero air pressure before opening any hydraulic connection.

Air pressure hazard: Drain air reservoirs before removing air line fittings. Verify with a gauge—do not rely on sound alone.

Brake fluid contamination: Use only the fluid type specified in the OEM service manual. Mixing DOT ratings or introducing petroleum products destroys rubber seals. Keep fluid in a sealed container.

Housekeeping: Brake fluid on the floor is a slip hazard. Use a drain pan and wipe spills immediately.

Key terms8

Air-over-hydraulic (AOH) brake systemA brake system that uses compressed air to actuate a hydraulic master cylinder, combining air power with hydraulic fluid delivery to wheel cylinders or calipers.
Foot valve (treadle valve)The driver-operated valve that meters air pressure from the reservoir to the AOH booster in proportion to pedal travel, providing brake modulation.
AOH booster/master cylinder assemblyThe combined unit that receives metered air on one side and converts that air force into hydraulic pressure through a linked air piston and hydraulic piston.
Control valveThe inlet-exhaust valve inside the AOH booster that opens to admit air to the air chamber during brake application and exhausts air to atmosphere when the pedal is released.
Pressure multiplication ratioThe ratio of the air piston area to the hydraulic piston area; determines how much the booster amplifies air pressure into hydraulic pressure.
Hydraulic line pressureThe fluid pressure, measured in psi, generated in the lines between the master cylinder and the wheel cylinders or calipers as a result of the booster's mechanical action.
Bleeder screwA small valve at each wheel cylinder or caliper that is opened to allow brake fluid and trapped air to escape during the bleeding procedure.
Pascal's LawThe principle that pressure applied to a confined fluid is transmitted equally in all directions; the physical basis for hydraulic pressure generation in any hydraulic system.

Worked examples4

1. An AOH booster has an air piston with an effective area of 20 square inches. The hydraulic piston bore gives a hydraulic piston area of 1.0 square inch. The foot valve delivers 60 psi of air to the booster. What hydraulic line pressure does the booster generate, assuming 100% mechanical efficiency?
  1. Step 1 – Find the air force acting on the air piston: Force = Air pressure × Air piston area = 60 psi × 20 in² = 1,200 lb.
  2. Step 2 – Find the hydraulic pressure produced by that force acting on the hydraulic piston: Hydraulic pressure = Force ÷ Hydraulic piston area = 1,200 lb ÷ 1.0 in² = 1,200 psi.
  3. Step 3 – State the pressure multiplication ratio for reference: Ratio = Air piston area ÷ Hydraulic piston area = 20 ÷ 1.0 = 20:1. This confirms that hydraulic pressure is 20 times the applied air pressure.
Answer: The booster generates 1,200 psi of hydraulic line pressure; the pressure multiplication ratio is 20:1.
2. Using the same booster (air piston area = 20 in², hydraulic piston area = 1.0 in²), the driver modulates the pedal so the foot valve delivers only 45 psi. What is the resulting hydraulic line pressure?
  1. Step 1 – Find the air force: Force = 45 psi × 20 in² = 900 lb.
  2. Step 2 – Find the hydraulic pressure: 900 lb ÷ 1.0 in² = 900 psi.
Answer: 900 psi of hydraulic line pressure, showing that reduced pedal effort (lower air pressure) produces proportionally lower hydraulic pressure, which is how the driver modulates braking force.
3. A different AOH booster has an air piston area of 16 in² and a hydraulic piston area of 0.8 in². What is the pressure multiplication ratio, and what hydraulic pressure results from 80 psi of delivered air pressure?
  1. Step 1 – Calculate the ratio: 16 in² ÷ 0.8 in² = 20:1.
  2. Step 2 – Calculate the air force: 80 psi × 16 in² = 1,280 lb.
  3. Step 3 – Calculate the hydraulic pressure: 1,280 lb ÷ 0.8 in² = 1,600 psi.
  4. Step 4 – Confirm using the ratio shortcut: 80 psi × 20 = 1,600 psi. ✓
Answer: The pressure multiplication ratio is 20:1 and the hydraulic line pressure is 1,600 psi.
4. A technician checks an AOH system and finds the air supply gauge reads 110 psi at the reservoir, but after tracing the foot valve, only 55 psi arrives at the booster air inlet during a full brake application. The air piston area is 20 in² and the hydraulic piston area is 1.0 in². What hydraulic pressure is actually produced, and by how many psi is it lower than it would be at full reservoir pressure?
  1. Step 1 – Calculate actual hydraulic pressure: 55 psi × 20 in² = 1,100 lb; 1,100 lb ÷ 1.0 in² = 1,100 psi.
  2. Step 2 – Calculate hydraulic pressure at full reservoir pressure (110 psi): 110 psi × 20 = 2,200 psi.
  3. Step 3 – Find the deficit: 2,200 psi − 1,100 psi = 1,100 psi less than normal.
  4. Step 4 – Interpret: A 50% drop in delivered air pressure causes a 50% drop in hydraulic pressure, indicating a significant foot valve or air line restriction that must be diagnosed before the vehicle is returned to service.
Answer: Actual hydraulic pressure is 1,100 psi, which is 1,100 psi below what full reservoir pressure would produce. The foot valve or supply line requires inspection.

The math in this lesson2

Learners compute pressure multiplication as air piston area over hydraulic piston area.
6.RP.3G-MG.2
Brush up in the math halls at newSchoolWorks.com:
In class: Cutaway demonstration: instructor shows an AOH booster/master cylinder assembly, identifies the air inlet, control valve, and hydraulic piston, and explains pressure multiplication ratio using a calculation example on the whiteboard. · System trace on a training medium-duty truck equipped with AOH: students follow air lines from the foot valve to the booster and hydraulic lines to each wheel cylinder, labeling a blank schematic as they walk the vehicle. · Bleeding procedure demonstration: instructor demonstrates correct sequence for bleeding hydraulic lines on an AOH system; students observe, note differences from a pure hydraulic system, and answer a short written question set.
Materials: Medium-duty training truck with AOH brakes, cutaway AOH booster/master cylinder, blank AOH schematic worksheets, hydraulic line wrenches, bleeder kit, clean brake fluid, drain pan, PPE (goggles, chemical-resistant gloves), OEM AOH service manual
Check: Completed and correctly labeled AOH schematic scored at 80% or higher against answer key

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Brakes – Air Brake System Components
1. What is the primary purpose of the air piston inside an AOH booster/master cylinder assembly?
Brakes – Hydraulic System Diagnosis and Repair
2. An AOH booster has an air piston area of 18 in² and a hydraulic piston area of 0.9 in². The foot valve delivers 70 psi to the booster. What is the resulting hydraulic line pressure?
Brakes – Hydraulic System Diagnosis and Repair
3. When bleeding the hydraulic lines on an AOH brake system, at which wheel should bleeding begin?
Brakes – Safety and Hazard Control
4. Before opening any hydraulic line fitting on an AOH brake system for repair, the technician must FIRST:
Brakes – Air Brake System Components
5. Which component meters the air pressure delivered to the AOH booster in proportion to how far the driver depresses the brake pedal?
Brakes – Hydraulic System Diagnosis and Repair
6. A technician notices the brake pedal feels spongy on a medium-duty truck with AOH brakes. The most likely cause is:
Brakes – Hydraulic System Theory
7. Pascal's Law is the physical principle that explains how AOH systems multiply pressure. Which statement correctly describes this law?
Brakes – Hydraulic System Diagnosis and Repair
8. During an AOH bleeding procedure, the master cylinder reservoir runs completely dry before all four wheels are bled. What is the correct next action?
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