What would cause trailer air brakes not to work?

Trailer air brakes commonly fail due to issues with the air supply, leaks, or faulty brake components. Without adequate pressure and proper control, brakes may not apply, release, or function as intended.

Air supply and pressure problems

These issues involve the air compressor, storage tanks, hoses, and valves that generate and maintain the air pressure needed to actuate the brake system.

• Low or no system air pressure due to a faulty tractor compressor, leaking or empty trailer air tanks, or a regulator failure that prevents the system from charging.


• Air leaks in supply lines or at connections (glad hands, hoses, fittings) that bleed off pressure faster than the system can replenish it.


• Moisture or water in the air lines or tanks, which can freeze in cold weather or cause corrosion and blockages.


• Faulty or sticking governor, pressure sensor, or control valve that prevents proper pressure build-up or maintains incorrect pressure levels.

In summary, the reliability of the brake actuation depends on maintaining adequate pressure, free-flowing air, and functioning pressure-control components.

Diagnostic note

Inspect the tractor’s air gauge, listen for audible leaks, drain moisture from tanks, and verify that the trailer line pressure reaches the manufacturer’s specified range before testing brakes.

Brake components and mechanical issues

Problems inside the brake assemblies or their actuators can stop brakes from applying, releasing, or modulating correctly.

• Damaged or leaking brake chambers or diaphragms, which reduce the force generated to apply the brakes.


• Stuck or worn pushrods, clevises, or slack adjusters that prevent proper shoe alignment or timely brake application.


• Worn brake shoes/linings or contaminated drums, resulting in reduced friction and braking effectiveness.


• Spring brakes (park/emergency) stuck in the engaged or released position, causing unintended braking or failure to release when commanded.

These issues require inspection of the entire brake assembly, including chambers, pushrods, and the condition of shoes and drums, followed by appropriate replacement or adjustment.

Diagnostic note

Check for signs of air leaks around brake chambers, listen for hissing when brakes are applied, and measure clearance and wear on shoes and drums. Perform a brake test with the axle jacked if safe to do so.

Electrical control and valve issues

Although primarily an air system, several electrical components and control electronics influence how the air brakes respond to driver input.

• Faulty or misadjusted trailer control valve, emergency valve, or combination valve that prevents proper brake application or release.


• Wiring faults, blown fuses, or ABS/ECU faults that interfere with valve operation or brake signal processing.


• Vent or purge valve failures that allow pressure to bleed off unintentionally even when the brakes should be released.

Electrical faults can mimic air-system problems, so diagnosis should include wiring checks and, where applicable, ABS/trailer ECU codes.

Diagnostic note

Use a diagnostic scanner if available, inspect trailer electrical connectors, test the brake control valve manually, and verify that the system maintains pressure with the engine running.

Couplings and mechanical connections

Connections between the tractor and trailer must be clean, secure, and properly seated; issues here can disrupt air flow entirely.

• Loose, damaged, or misaligned gladhands causing air leaks or failure to build pressure in the trailer line.


• Cracked or kinked air hoses or damaged fittings at the gladhand connections or valve box.


• Blocked or malfunctioning trailer valve body preventing proper release or application of brakes.

Regular inspection of the gladhand couplers, hoses, and valve bodies helps prevent mis-seating or leaks that render the brakes ineffective.

Summary

Trailer air brakes depend on a charged, clean air system and well-maintained brake hardware. The most common causes of brakes not working are low pressure or leaks in the air supply, faulty brake chambers or actuators, defective valves or control electronics, and damaged connections between the tractor and trailer. Regular preventive maintenance, prompt repairs, and careful trip inspections are essential for safety and compliance.

If brake problems are suspected, stop driving, diagnose safely, and seek professional repair to ensure the system meets regulatory requirements and safety standards.

Why are my air brakes not working?

If air brakes won't release despite normal compressor pressure, check the brake chambers for stuck diaphragms or damaged return springs. Inspect the air release valve and brake control valve for proper operation and air leaks. Ensure the brake release button and related solenoids function correctly.

How to tell if an air chamber is bad?

Reduced Braking Power: One of the most noticeable signs of a failing air brake chamber is a significant reduction in braking power. The vehicle may take longer to come to a stop, and the brakes may feel less responsive.

How to test a trailer brake valve?

Press in the red trailer parking brake valve and listen for audible leaks. Press and hold the foot valve (brake pedal) while watching the air pressure gauges. Once the air pressure stabilizes on the gauges, hold the brake pedal for one minute watching for any pressure loss.

Why are my trailer brakes not engaging?

Trailer brakes not engaging can be caused by electrical issues like a blown fuse, a faulty brake controller, or bad connections in the wiring harness and plug, or mechanical problems such as worn brake shoes or magnets or improper brake adjustment. To troubleshoot, start by checking the electrical system from the tow vehicle to the trailer, including fuses and plugs, and then inspect the mechanical components by testing the wheels and brake assemblies.
 
This video explains how to troubleshoot trailer brakes, starting with checking the ground connection: 56setrailerYouTube · Dec 30, 2024
Electrical troubleshooting

• Check the tow vehicle's brake controller: Make sure it is powered on and working correctly. You can often test it by manually activating the brakes to see if it sends a signal. 
• Inspect the trailer plug and socket: Look for corrosion, bent pins, or loose connections. Clean the contacts with a wire brush or electrical contact cleaner. 
• Verify the trailer's wiring: Trace the wiring from the plug to the brake assemblies on each wheel. Look for any signs of damage, such as pinched wires, frayed insulation, or exposed conductors. 
• Test the ground connection: A poor or broken ground is a common cause of brake failure. Check that the ground wire is securely connected to a clean, bare metal surface on both the tow vehicle and the trailer chassis. 
• Check for blown fuses: The tow vehicle and trailer may have fuses or circuit breakers for the brake system that could be blown or tripped. 

Mechanical and component troubleshooting

• Test the brakes with the breakaway switch: Lift a wheel off the ground, then pull the breakaway switch cable. If the brakes are working correctly, the wheel should lock up and be difficult to turn. 
• Inspect the brake components: If the wheel locks when the breakaway is pulled, the issue is likely with the wiring or controller. If not, remove the brake drum to inspect the brake shoes and magnets for wear or damage. 
• Check the brake adjustment: Electric drum brakes need to be adjusted properly. If they are too loose or too tight, they won't perform correctly. 
• Look for grease contamination: If the wheel seal has failed, grease can get on the brake shoes, significantly reducing braking force. 

This video demonstrates how to adjust the brake controller: 1mThe TrailerSmithYouTube · Sep 21, 2025
Testing voltage with a multimeter 

• Use a multimeter to check for voltage at the trailer plug and at the brake assemblies.
• If you are getting voltage at the plug but not at the assemblies, the problem is likely in the trailer's wiring harness.
• If the voltage is lower than expected at the assemblies, there might be a poor connection or a corroded ground somewhere along the circuit.