Common Air Brake and Valve Leaks and How To Fix Them

Air brake leaks put semitrucks out of commission quickly. Downtime eats into profits and productivity for semitruck drivers and fleet managers.

Ignoring a hissing noise or a dropping pressure gauge costs you money every minute that rig sits idle in the shop instead of hauling freight. Worn seals and gaskets, rust, cracked air lines, and worn-out valve assemblies often stand at the root of these issues. Learn how to address common air brake and valve leaks, as well as how to fix them.

The Valves You Need To Watch

Air brake systems rely on numerous valves. Here are the most common sources of leaks:

• The foot valve works with the brake pedal, metering air to the service brakes. Listen for a hissing sound in the cab when pressing or releasing the brakes.
• The quick-release valve, located near the front brake chambers, vents air rapidly when releasing the brakes and speeds up disengagement. A failed quick-release valve may leak air all the time or trap air, causing brake drag.
• The relay valve is near the back axles and lets the rear brakes react quickly. Delays in rear braking or air escaping from the valve, even when the brakes are off, indicate issues.
• The spring brake valve controls the parking and emergency brakes. Any hissing or failure to hold means you should inspect this part.
• The inversion valve guarantees that the spring brakes will activate if the service brake pressure drops severely.
• The pressure protection valve protects the air brake system by closing off air loss if an accessory, like the horn or suspension springs, has a major leak.

Troubleshooting the Foot Brake Valve

The foot valve frequently develops leaks due to dirt and debris entering the system. When the driver isn’t applying the brakes and air escapes from the exhaust port, the internal inlet valve isn’t sitting against the piston correctly.

Debris lodged between the seal and the seat allows air to pass through continuously. Sometimes, you can clear this by firmly applying and releasing the brake pedal several times to dislodge the particle. If the leaking persists, deteriorated or damaged internal seals are the likely culprits.

Check if the leak occurs only when applying the brakes. An exhaust leak during application suggests a bad exhaust seat or O-ring within the valve assembly. In most cases, rebuilding these valves consumes more time than it is worth, so replacing the entire unit is the standard repair.

Addressing Dash Control Valve Leaks

The parking brake (yellow) and trailer supply (red) knobs on the dashboard are common failure points. You might hear a hissing sound directly behind the dash panel, or the knobs might pop out unexpectedly. This usually points to worn O-rings on the spool valve inside the manifold.

These valves cycle thousands of times over the life of a truck, causing the rubber O-rings to flatten or crack. When these seals fail, air bypasses the spool and vents out the exhaust port or around the shaft of the button itself. Spraying soapy water around the shaft of the knob will confirm if the top seal has failed.

Replacing the O-rings serves as a temporary fix, but the plastic bodies of modern valves can also wear out. Installing a new dash control module solves the problem permanently.

Diagnosing Relay Valve Issues

Relay valves mount near the rear axles and speed up the application and release of the rear brakes. A leak at the relay valve exhaust port often confuses mechanics because the valve itself might not be the culprit. You need to determine if the air is coming from the valve or back-feeding from a brake chamber.

Disconnect the service line from the relay valve while the parking brakes are released. If air rushes out of the line coming from the brake chamber, the internal seal in the spring brake chamber has failed. This allows hold-off air to leak into the service side and back through the relay valve exhaust.

If no air comes from the line, the issue lies within the relay valve itself. A damaged internal piston or a piece of debris on the seat will cause a constant leak. Replacing the relay valve is necessary when there are damaged internal components.

Fixing Air Dryer Purge Valve Leaks

The air dryer removes moisture and oil from the compressed air system to protect downstream components. The purge valve at the bottom of the dryer expels these contaminants when the compressor unloads. A common issue arises when the purge valve stays open or leaks constantly after the purge cycle finishes.

This usually happens because sludge or carbon build-up prevents the valve from closing completely. In winter, ice can also block the valve if the heater element fails to function. You can often fix this without replacing the entire air dryer assembly.

Installing a purge valve rebuild kit resolves the issue most of the time. The kit typically includes new O-rings, a piston, and a spring to restore the sealing capability. Clean the seat thoroughly before installing the new parts to prevent immediate recurrence of the leak.

Quick-Release Valve Malfunctions

Quick-release valves allow air to exhaust rapidly from the brake chambers when the driver lets off the pedal. You will find these installed on the front axle or near the rear brake chambers. A continuous leak from the exhaust port often indicates a ruptured diaphragm.

The rubber diaphragm inside flexes every time the brakes are applied, leading to fatigue over time. When it cracks, supply air escapes directly out of the exhaust port instead of going to the brake chambers. This reduces braking force and creates a noticeable leak.

Repairing this is straightforward. Unscrew the top cover of the valve to access the diaphragm directly. Replace the rubber component and the cover seal, then tighten the bolts evenly to restore function.

Handling Compressor Governor Issues

The governor controls when the air compressor pumps air into the tanks and when it stops. If you hear the safety valve popping off on the air dryer or the supply tank, the governor might be too high. Conversely, if the system never reaches 120 psi, the governor might not signal to the compressor to load.

Leaks at the governor mounting gasket or the sensing lines cause erratic compressor cycling. Check the small air lines running between the governor and the air dryer for chafing or cracks. A leak in the unloader line will prevent the compressor from stopping, leading to dangerous over-pressure situations.

Adjusting the cut-out pressure is possible on most governors by removing the top cap and turning the adjustment screw. However, if the internal diaphragm leaks, replacing the unit is the only safe option.

Protect Your Fleet With the Help of Van Horn Truck Parts

Addressing common air brake and valve leaks promptly demonstrates your commitment to safety and efficiency. For high-quality replacement air brake valves that keep your trucks running efficiently, trust Van Horn Truck Parts. Our extensive inventory covers everything from dash valves to air dryers. Visit our online catalog today to find the exact parts you need to seal up your system.