Do air brakes run out of air?

They can lose pressure, but the systems are designed to fail safe: on trucks and buses, low air pressure causes powerful spring brakes to apply; on trains, a loss of air automatically applies the brakes. Under normal operation, an engine-driven compressor continually replenishes the air supply, so the brakes do not “run out” during regular driving.

How modern air brake systems work

Air brakes on heavy road vehicles and trains use compressed air stored in tanks. A compressor charges reservoirs to a set pressure, and governors and valves manage when air is used, stored, or vented. The design philosophy is fail-safe: if pressure drops too low, braking force is applied rather than lost, preventing runaway situations.

Road vehicles (trucks, buses, trailers)

Heavy vehicles use two complementary systems. The service brakes use air pressure to apply friction at the wheels when the pedal is pressed. The parking/emergency brakes are spring-powered; air pressure holds these springs off during normal driving. If system pressure falls—because of leaks, compressor failure, or repeated applications without recharge—the springs apply the brakes, stopping or holding the vehicle. Typical governor settings cut in around 100 psi and cut out around 120–135 psi; low-air warnings usually activate near 60 psi, and spring brakes often begin to apply between roughly 20–45 psi, depending on the vehicle.

Rail vehicles (trains)

Freight and passenger trains use the Westinghouse-style automatic air brake. The trainline (brake pipe) is pressurized—commonly around 90 psi—and air in each car’s reservoir is used to apply brakes when the pipe pressure is reduced. Releasing the brakes requires restoring pressure. If the pipe breaks or the compressor fails, pressure falls and the brakes apply automatically, bringing the train to a stop.

What “running out of air” looks like in practice

Drivers rarely “run out of air” in normal service because the compressor replenishes pressure while the engine runs. Problems arise when leaks exceed compressor output, the compressor or air dryer fails, air lines freeze, or the vehicle is repeatedly braked without time to recharge (for example, riding the brakes on long descents). The system’s warnings and automatic actions are designed to alert the driver and stop the vehicle before braking capability is lost.

Common causes of air pressure loss

Multiple factors can reduce air pressure in a braking system. Understanding them helps drivers and technicians identify issues before they become hazardous.

• Air leaks in hoses, fittings, reservoir valves, or brake chambers
• Compressor or governor malfunction reducing charging capacity
• Air dryer problems causing moisture accumulation and winter freeze-ups
• Excessive brake applications without recharge time, especially on long grades
• Faulty check valves or quick-release valves allowing backflow or slow recharge
• Poor maintenance: undrained tanks, worn seals, or damaged gladhand gaskets (for combinations)

While many of these faults develop gradually and trigger early warnings, severe leaks or component failures can produce rapid pressure loss that demands immediate, controlled stopping.

Warning signs and system thresholds

Air brake systems provide standardized alerts and behaviors as pressure falls. Knowing the typical thresholds helps drivers respond appropriately.

• Low-air warning (light/buzzer) activates around 60 psi (varies by regulation and vehicle)
• Service brake feel changes: longer pedal travel or weaker response as pressure decays
• Spring brakes begin to apply automatically, often between roughly 20–45 psi
• Governor cycle: compressor cuts in near 100 psi and cuts out around 120–135 psi
• For trains, any unexpected sustained drop in brake-pipe pressure leads to a service or emergency application

These thresholds are designed to give the operator time to get off the roadway or prepare the train to stop before full automatic application occurs.

Best practices to avoid low-air situations

Simple habits and maintenance checks greatly reduce the risk of air depletion and unintended stops.

• Perform pre-trip air system checks: build-up rate, low-air warning, and leak-down tests
• Drain reservoirs as required and ensure the air dryer is functioning
• Use proper downhill techniques: select low gears early and rely on engine/exhaust brakes, not continuous pedal braking
• Heed low-air warnings: do not “pump” the brakes; pull over safely and let pressure rebuild
• In cold weather, prevent moisture accumulation and consider anti-freeze protocols per manufacturer guidance
• For combinations, inspect gladhands, seals, and trailer supply valves before departure

These measures help keep pressure within target ranges and ensure the fail-safe features work as intended if problems arise.

What to do if pressure drops while driving

If the low-air warning activates or the pedal feel changes, treat it as an immediate safety issue. Maintain lane control, signal, and move to a safe stopping area. Avoid repeated brake applications that can accelerate pressure loss. Allow the compressor to rebuild pressure and investigate the cause before proceeding; if the problem persists, call for service.

Bottom line

Air brakes do not normally “run out of air” because an onboard compressor keeps the reservoirs charged. If air pressure does fall—due to leaks, failures, or misuse—both truck and train systems are engineered to apply the brakes automatically. The result is not brake loss but an enforced stop, provided operators heed warnings and follow safe procedures.

Summary

Air brake systems are built to fail safe. On trucks and buses, low pressure releases spring-powered brakes that apply automatically; on trains, a drop in brake-pipe pressure triggers braking. While reservoirs can be depleted by leaks or improper technique, regular maintenance, proper downhill driving, and prompt response to low-air warnings prevent hazardous situations and keep the system operating as designed.

Can air brakes run out of air?

The supply of air is unlimited, so the brake system can never run out of its operating fluid, as hydraulic brakes can.

How does an air brake work?

Air brakes work by using compressed air from a reservoir tank to push brake chambers and their attached S-cam mechanism, which forces brake shoes against a spinning drum or brake pads against a disc to slow the vehicle. When the driver releases the brake pedal, the air is released, a spring pulls the shoes away, and the wheels can turn freely again. 
Here’s a breakdown of the process:

1. 1. Compressed Air Production: . Opens in new tabAn engine-driven compressor continuously pressurizes air, which is then stored in air tanks. 
2. 2. Air Control Valve (Brake Pedal): . Opens in new tabWhen the driver presses the brake pedal, the air control valve releases a regulated amount of air from the reservoir. The harder the pedal is pressed, the more air is released. 
3. 3. Brake Chamber Activation: . Opens in new tabThe compressed air flows to the brake chambers at each wheel. Inside, the air pushes on a diaphragm, which moves a rod outward. 
4. 4. S-cam Rotation: . Opens in new tabThis rod is connected to a slack adjuster, which then rotates a camshaft with an S-shaped cam at the end. 
5. 5. Friction for Stopping: . Opens in new tabThe rotating S-cam forces the brake shoes outwards, pressing them against the inside of the brake drum. In disc brake systems, the S-cam pushes a pushrod which engages the brake discs, slowing the vehicle. 
6. 6. Brake Release: . Opens in new tabWhen the driver releases the brake pedal, the air in the brake chambers is exhausted. 
7. 7. Spring Return: . Opens in new tabA powerful spring inside the brake chamber pulls the brake shoes or pads away from the drum or disc, allowing the wheels to roll freely. 

Why Air Brakes Are Used:

• Reliability: Air brake systems are very reliable and have built-in failsafe features; if the air pressure drops too low, the brakes will automatically apply, preventing a runaway vehicle. 
• Durability: They are generally more durable, experience less brake fade from overheating, and have no risk of fluid leaks. 
• Power: They provide the necessary stopping power for heavy commercial vehicles that are difficult to stop with hydraulic systems. 

Will air work itself out of brakes?

No, air in brake lines will not go away on its own; it must be manually removed through a process called bleeding the brakes. Air in the lines prevents the proper transfer of hydraulic force, leading to a spongy, unresponsive brake pedal and significantly reduced braking power. Driving with air in the brake lines is dangerous, as it can result in complete brake failure. 
This video demonstrates the process of bleeding brakes: 54sBuddy’s DIYYouTube · Dec 12, 2020
Why air needs to be bled:

• Reduced braking power: Air is compressible, unlike brake fluid, so when you press the brake pedal, you are compressing the air instead of transferring that force to the calipers. 
• Spongy brake pedal: This compression results in a “squishy” or soft brake pedal that feels unresponsive. 
• Brake failure risk: Over time, the problem can worsen, leading to a significant lack of pressure and potentially a complete loss of braking ability. 

How to remove air (brake bleeding):

1. 1. Locate the bleeder screws: These are small fittings, often on the brake calipers or wheel cylinders, where the air is trapped. 
2. 2. Attach a hose: A clear tube is often attached to the bleeder screw, with the other end placed in a container. 
3. 3. Pump the pedal: Slowly pump the brake pedal several times to build pressure, keeping the master cylinder reservoir topped off to avoid drawing in more air. 
4. 4. Open the bleeder screw: While holding the pedal down, briefly open the bleeder screw to release the compressed air and some brake fluid. 
5. 5. Close the bleeder screw: Before releasing the pedal, close the bleeder screw to prevent more air from being sucked back into the line. 
6. 6. Repeat: Continue this process at each wheel until no more air bubbles appear in the fluid, and the fluid runs clear. 

You can watch this video for a step-by-step guide on bleeding brakes: 35sBevins BuildsYouTube · Jul 4, 2020
Important Safety Note:

• If you are unsure how to bleed your brakes, it is best to have the work performed by a professional. 
• The vehicle’s brake system is a critical safety component, and any issues should be addressed promptly. 

What is the air loss rate for a CDL?

For the CDL Commercial Driver’s License air brake pre-trip inspection, the acceptable air pressure loss rate for a single vehicle is no more than 2 psi in the first minute after the initial pressure drop, and no more than 3 psi in one minute for a combination vehicle (tractor and one or more trailers). If the air pressure drops more than this limit, it indicates a significant air leak, and the vehicle should not be driven until the leaks are repaired. 
How to Perform the Air Leakage Rate Test 

1. Fully charge the air system: Start the engine and build the air pressure in the tanks to the “governor cut-off” point, typically 120-140 PSI.
2. Turn off the engine: Shut off the engine and fully apply the service (foot) brake.
3. Release the brakes: With the engine off, release both the parking brake and the tractor protection valve.
4. Monitor pressure: Time the air pressure drop for one minute.

Acceptable Pressure Loss Limits

• Single Vehicle: The pressure should not drop more than 2 PSI in the first minute. 
• Combination Vehicle: The pressure should not drop more than 3 PSI in the first minute. 

What to do if the Leak is Too High 

• If the air pressure falls more than the allowable limit (3 PSI for a single vehicle or 4 PSI for a combination vehicle, as noted in some sources after the initial drop), the air loss rate is too high.
• You must check for air leaks and have them repaired before driving the vehicle, as a significant leak can lead to a loss of brakes while driving.