How to detect a leak in an air brake system?

Searching for leaks in air brake systems often turns into randomly checking connections with soapy water. However, a systematic approach is both faster and more reliable.

Step 1: Static pressure drop test

Charge the system to the cut-out pressure, stop the engine, and record the pressure drop by waiting for a specified time without releasing the parking brake. Measure the pressure drop with the brake pedal both applied and released — the difference between the two helps narrow down whether the leak is in the service brake circuit or the supply side.

Step 2: Isolating the circuits

In modern vehicles, circuits are divided into four zones: front brakes, rear brakes, parking/auxiliary, and trailer supply. By isolating the circuits one by one, determine which one continues to show a pressure drop. This step reduces the number of connections to check to a tenth.

Step 3: Component inspection

Once the leaking circuit is identified, it is time to inspect the components. The most common points are:

• Brake chamber and diaphragm tears
• Internal leaks in the quick release valve and relay valve
• Worn seals at fittings and hose connections
• Air dryer purge valve

Since internal leaks are not visible from the outside, they cannot be found with a soap test; they can only be detected through circuit isolation. This is the main reason why step 2 should never be skipped.

Common mistake

Failing to repeat the test after a leak is found and fixed. It is common to have multiple leaks in a system; once the first is fixed, the second becomes measurable. The test must be repeated until the pressure drop falls to an acceptable level.