How to Use Air Brakes on a Truck

To use air brakes on a truck safely, build system pressure to governor cut-out (about 120–135 psi), confirm air system health with pre-trip leak and warning tests, brake progressively on level roads, use snub-braking with engine retarder on downgrades, monitor air gauges continuously, and apply spring (parking) brakes when stopped. Air brakes behave differently from hydraulic systems due to air “lag,” heat sensitivity, and automatic spring brakes; understanding these differences—and the role of ABS—prevents fade, jackknifes, and runaways. This guide explains the essentials recognized in commercial driver training and industry practice.

What an Air Brake System Does—and Why It’s Different

Air brakes use compressed air to apply service brakes and powerful spring brakes for parking and emergency application. Most highway tractors use dual-circuit systems with ABS, air dryers, and automatic slack adjusters. Typical working pressure ranges around 100–125 psi, with the governor cutting out near 120–135 psi and cutting back in near 100 psi. A low-air warning activates around 55–60 psi; spring brakes normally “pop out” (apply) at roughly 20–45 psi. Unlike hydraulics, air systems have a fraction-of-a-second delay (lag) from pedal to brake application, and they are sensitive to heat buildup on long grades.

Pre-Trip: Proving the System Is Safe

Before driving, verify that the air system builds, holds, and warns correctly. These checks are a cornerstone of CDL training and can prevent catastrophic failures.

1. Secure the vehicle: Park level, chock wheels, apply parking brakes (yellow tractor and, if a combination, red trailer supply).
2. Build pressure: Start engine and build to governor cut-out (about 120–135 psi). Note primary and secondary gauges reach normal range.
3. Key-on/engine-off: Shut off engine; leave key on so gauges and warnings function.
4. Release parking brakes: Push in parking valves to release spring brakes (only if chocked).
5. Static leak test: With no brake pedal applied, observe pressure drop for one minute; max loss is 2 psi (single vehicle) or 3 psi (combination).
6. Applied leak test: Apply and hold a firm service-brake application; after stabilization, loss should not exceed 3 psi (single) or 4 psi (combination) in one minute.
7. Low-air warning: Fan the brake pedal; the buzzer/light must activate by about 55–60 psi.
8. Spring brake pop-out: Continue fanning; parking valves should pop out between roughly 20–45 psi.
9. Rebuild air and roll check: Restore full pressure, then at low speed perform a gentle service-brake stop to confirm even braking and no pull.
10. Parking brake checks: Tug test tractor and trailer parking brakes separately (on level ground at low throttle) to confirm they hold.
11. ABS indicator: Ensure the ABS light(s) illuminate on power-up and then go out; a light that stays on signals a fault.

If any threshold is not met—excessive leakage, late warnings, or non-functioning ABS—repair before operating. Always drain moisture from tanks daily unless automatic drains are verified functional.

Starting Off and Reading Your Gauges

Don’t move until both primary and secondary air pressure gauges are within normal range and stable. The application pressure gauge shows how much pressure you’re using; rising application pressure at the same road load can hint at fading brakes, dragging components, or an air issue. Build a habit of scanning air pressure like you scan mirrors.

Normal Braking on Level Roads

Use smooth, progressive applications—press the pedal just enough to slow, then ease off as you reach target speed. Remember brake lag; begin braking a bit earlier than you would in a car. Maintain long following distances and keep space cushions large so you can use light applications that minimize heat buildup.

Downhill Control: Engine Retarder + Snub Braking

Long or steep grades demand a plan. The goal is to control speed with the engine and brief, firm brake applications rather than riding the brakes and overheating them.

1. Pick a safe speed before the grade and select the proper gear beforehand; do not upshift once committed to the hill.
2. Use the engine/exhaust brake if traction is good; turn it off on slippery surfaces to avoid drive-axle skids.
3. Apply “snub braking”: when speed creeps to the upper limit of your chosen range, apply the brakes firmly for about 3–5 seconds to drop 5 mph (about 8 km/h), then release to let brakes cool.
4. Avoid riding the brakes; continuous light pressure overheats drums/rotors and causes fade.
5. Watch for fade signs: longer pedal travel, rising application pressure for the same effect, burning smell, or smoke. If detected, downshift more, use lower speed, or stop in a safe area to cool.

Effective grade control mixes gearing with intermittent, decisive applications. If brakes overheat or fade, stop as soon as it’s safe—continuing risks total loss of braking.

ABS, Emergency Stops, and Skid Control

Modern tractors and trailers feature ABS, which helps you steer during heavy braking. Your technique should match the equipment.

• With ABS: Apply firm, steady brake pressure; do not pump the brakes. Expect pedal vibration or rapid modulation—this is normal.
• Without ABS: Use controlled braking—steady pedal pressure just short of lockup—or stab braking (full apply until lock, release to roll, reapply) only if necessary and trained.
• Steer where you want to go; keep the vehicle straight under hard braking to avoid jackknife risk.
• Never use the trailer hand valve (“trolley valve”) for normal braking or while driving; it can cause a trailer skid. Reserve it for low-speed brake checks only.
• On slippery roads, ease off the engine brake/retarder to prevent drive-wheel skids.

ABS won’t always shorten stopping distance, but it improves control. Your priority in an emergency is maintaining directional stability while maximizing available traction.

Parking, Coupling, and Leaving the Truck

Parking and yard movements involve different air controls and risks than highway driving. Handle valves deliberately and avoid relying on air pressure to “hold” a parked rig.

• For parking, use spring brakes (yellow tractor, red trailer supply pulled out); chock wheels on grades or when working under/around the vehicle.
• Do not park with the trailer hand valve; air can bleed off and release brakes.
• In extreme cold after wet conditions, brakes can freeze. If needed, chock wheels and follow cold-weather procedures; avoid setting hot, wet brakes until cooled and dried when practical.
• When coupling, charge the trailer air system and perform tug tests; at walking speed in a safe area, you may momentarily use the hand valve to verify trailer service brakes apply—then return it to neutral.

Use a consistent routine: set spring brakes, chock as needed, and verify tug tests during coupling. Vigilance here prevents rollaways and yard incidents.

Weather, Heat, and Brake Fade

Air brakes are robust, but friction components are vulnerable to heat and poor traction. Anticipation and restraint are key in adverse conditions.

• Wet/icy roads: Lengthen following distance, disable engine brake on slick surfaces, and use gentler applications.
• High heat or heavy loads: Favor lower gears and snub braking on grades; stop to cool if signs of fade appear.
• Dust, mud, or water crossings: Expect reduced initial braking; dry brakes with light applications once clear and safe.

Adjusting technique to conditions preserves braking capacity and reduces risk of skids or fade-related failures.

Maintenance and Daily Care You Control

While most service is for technicians, drivers influence brake safety through routine care and inspections.

• Drain air tanks daily to remove moisture unless equipped with verified automatic drains; service the air dryer per schedule.
• Inspect hoses, gladhand seals, and lines for leaks, abrasion, or kinks; listen for leaks during walk-arounds.
• Check slack adjusters for excessive free play; if a pushrod moves much more than about an inch at the slack adjuster, report it. Do not adjust automatic slack adjusters yourself—have a qualified technician inspect.
• Monitor wheel ends for heat, smell, or smoke after heavy braking; a quick hand-hover (without touching) near hubs can hint at overheated brakes.
• Report any ABS warning lights, late low-air warnings, slow air build, or unusual application pressures immediately.

Simple habits—draining tanks, eyes-and-ears inspections, and timely reports—prevent small issues from becoming safety-critical failures.

Common Mistakes to Avoid

Avoid these frequent errors that lead to incidents, violations, or expensive repairs.

• Riding brakes on long downgrades instead of using snub braking and lower gears.
• Skipping leak, warning, and pop-out tests before a trip.
• Ignoring a low-air warning or continuing after a rising application-pressure trend.
• Using the engine brake on slick surfaces, causing drive-axle skids.
• Applying or relying on the trailer hand valve while moving or for parking.
• Pumping ABS brakes, which reduces effectiveness and control.

Steering clear of these pitfalls protects you, your equipment, and everyone sharing the road.

Standards and Where to Learn More

Specific thresholds and procedures can vary with jurisdiction and equipment. The figures above align with widely taught CDL guidance in North America: governor cut-out around 120–135 psi, low-air warning around 55–60 psi, spring brake pop-out roughly 20–45 psi, and leak-test limits of 2/3 psi (static) and 3/4 psi (applied) for single/combination vehicles. Always follow your company policies, the vehicle manufacturer’s manual, and your local/commercial driver handbook. Formal training and supervised practice are essential before operating air-braked vehicles in traffic or on grades.

Summary

Using air brakes correctly means confirming system health before moving, braking smoothly on level roads, controlling speed on grades with gearing and snub braking, leveraging ABS without pumping, and applying spring brakes and chocks when parked. Monitor gauges, respect low-air warnings, and adjust technique for weather and load. With disciplined checks and conservative driving, air brakes deliver powerful, reliable stopping for heavy trucks.

How to properly stop with air brakes?

Push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, don’t push the clutch in until the engine rpm is down close to idle. When stopped, select a starting gear.

How do air brakes work on trucks?

Truck air brakes work by using compressed air from the engine’s air compressor to store and deliver pressure to the wheels, which activates the brake system. When the driver presses the brake pedal, air from storage tanks is sent to the brake chambers at the wheels, moving pistons that push the brake shoes or pads against the drums or rotors, creating friction to slow the truck. If the air pressure drops, a safety feature uses spring force to apply the brakes automatically, and a dryer removes moisture from the compressed air to prevent it from freezing and causing issues.
 
Key Components

• Air Compressor: Driven by the engine, it compresses ambient air into the system. 
• Air Tanks/Reservoirs: Store the compressed air until it’s needed. 
• Air Dryer: Removes moisture from the compressed air to prevent freezing. 
• Foot Valve/Brake Pedal: A control that allows the driver to send air from the tanks to the brakes. 
• Brake Chambers: Located at the wheels, these contain diaphragms that are pushed by air pressure to activate the brakes. 
• Brake Shoes and Drums (or Disc Brakes): The components that create friction against the wheel’s drum or rotor to slow the vehicle. 

This video explains how air brakes work in detail, including the role of the air compressor and brake chambers: 20s3D Casual LearningYouTube · Jun 11, 2025
How the System Works

1. Air Compression & Storage: The engine’s compressor pumps air into the storage tanks, building up pressure. 
2. Brake Application: When the driver presses the brake pedal, the foot valve sends compressed air from the tanks to the brake chambers at each wheel. 
3. Friction Creation: The air pressure pushes a diaphragm or piston inside the brake chamber. This action activates a slack adjuster and pushrod, which rotates an S-cam (in drum brakes) or a power screw (in disc brakes). This component then presses the brake shoes against the inside of the drum or the pads against the rotor, slowing the vehicle. 
4. Brake Release: Releasing the brake pedal allows the air to escape from the brake chambers, and springs push the diaphragm or piston back, releasing the brake shoes or pads. 
5. Safety & Parking: The system is designed to apply brakes automatically if air pressure gets too low, using the force of a spring in a spring brake chamber. The parking brake knob, when pulled out, exhausts air from these chambers, allowing the spring force to set the brakes. 

You can watch this video to see how the spring brake applies the brakes when air pressure is released: 37sSmart Drive TestYouTube · Apr 4, 2017

Do you push or pull to release air brakes?

To release air brakes, you typically push the control knob or lever in. When you push the knob in, it supplies air to the air chambers, which pushes against the springs and allows the brakes to release. Conversely, pulling the knob out engages the parking brakes by removing this air pressure, allowing the springs to apply the brakes. 
Summary of Air Brake Control 

• To Engage Brakes (Set): Pull the yellow diamond-shaped control knob out.
• To Release Brakes: Push the yellow diamond-shaped control knob in.

Why This Works

• Spring Brakes: The parking brakes (also called spring brakes) are held in the “on” position by powerful springs. 
• Air Pressure: Air pressure is used to overcome these springs and keep the brakes in the “off” (released) position. 
• Control Knob: The push-pull control knob is the mechanism that controls the release and application of this air pressure to the parking brakes. 

How to use air brakes for beginners?

You engage the brakes by pushing down the brake pedal (It is also called a foot valve or treadle valve). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes.