Roughly 26,400 vehicle accidents each year are caused by faulty brakes.
Fortunately, it's simple to avoid ending up as one of those statistics. As you drive a truck, bus, or similar heavy-duty vehicle, federal CDL laws require routine checks for signs of air brake system malfunction or poor performance.
If signs indicate a potential problem, it’s equally straightforward to troubleshoot and find the underlying issue. That said, straightforward doesn’t necessarily mean “easy.”
In this guide, the experts at Point Spring & Driveshaft Co. walk you through different methods of air brake troubleshooting step-by-step. Then, we offer insights into how you might solve different problems once they're identified.
When To Start Air Brake System Troubleshooting?
It’s important to begin troubleshooting at the very first sign your brakes might have a problem. This is both important for safety and cost-efficiency. Often, catching a problem earlier makes solving it less expensive.
11 Signs Your Truck’s Air Brakes Are In Trouble
If you see any of these eleven signs, it’s wise to start troubleshooting your air brake system.
The brake warning light on your dashboard is on
Your break’s response lags
The brake feels “soft,” easily pushed to the floor
Brake is “locked,” doesn’t respond to pressure or release
Squealing, clinking, or grinding brake sounds
Unexplained vibrations
Truck veers to one side
Air pressure takes too long to build up after the truck starts
Gauge shows low or losing air pressure
Inconsistent brake force or brake pressure
There’s a “burning” smell during or immediately after braking
What Causes Air Brake Issues? 10 Common Culprits
Air brakes can lose functionality for a number of reasons. If the system’s components are the wrong type, ill-fitting, or misaligned, leaks and corrosion will spring up. If any parts become damaged or contaminated, or if the system isn’t well-lubricated, air brakes can start malfunctioning.
To troubleshoot effectively, it's vital to inspect your air brake system for signs of the ten most common causes of air brake issues.
Misalignment
Wrong-size components
Insufficient lubrication
Ungreased or under-greased camshafts, slack adjusters
Too much friction causes components to both “seize” and overheat
Leaks
Damaged or inappropriate seals, gaskets
Punctured or damaged diaphragms within brake chamber
Cracks in air lines
Loose, corroded, or faulty fittings / components
Other component wear and tear, corrosion
Insufficient air compression
Overheating
Decreases necessary brake pad friction
Warps brake drums
Brake shoe jam
Shoe component stuck in “partially applied” position within drum
Air tank contamination
Typically contaminated by oil, grit
This can sometimes be swiftly resolved through alcohol drying
Troubleshooting: Determine The Root Cause of Air Brake Issues
To resolve air brake dysfunction, first diagnose the core problem. Prepare to assess the vehicle’s:
Service Brake System
Parking Brake System
Emergency Brake System
To ensure nothing is missed, experts typically use model-based or data-driven fault diagnostic methods. Often, both methods are used as-needed.
In practice, this can mean both engaging in manual best practices, like measuring the push-rod offset by hand, while simultaneously using best-in-class digital tools to gather and analyze data.
Both diagnostic strategies have a lot to offer. Here’s how our experts approach these two, distinct methods.
Model-Based Diagnostics
Model-based diagnostic methods determine the source of a problem by using a model of the vehicle’s air brake system’s standard or expected behavior (i.e. function, flow). A technician both visually inspects and tests each aspect of the system, comparing and contrasting it with the model.
A professional team can enact model-based diagnostics using a standard air brake system component layout, specific to the model of truck or bus they're assessing. Typically, the model includes the layout, flow, and function of all system parts, including:
Air compressor and compressor governing components
Air tanks, tank drains, and reservoirs
Brake pedal
Foundation brakes (at wheel)
Brake shoes
Brake drums / rotors
Brake linings
Alt. foundation brakes
S-Cam brakes
Wedge brakes
Disc brakes
Brake chambers / slack adjusters
Pressure gauges and warning mechanisms
Brake lights
Spring brakes (emergency and parking brakes)
Parking Brake controls
Safety valves
Dual parking control valve
Air brake relay valve
Air brake foot valve
Air brake quick release valve
Modulating valve
Air tank relief valve
Front brake limiting valve
Antilock Braking System (ABS)
State, federal, and professional industry organizations have established standard system models for model-based diagnostics.
Many of these organizations have likewise set standards for thorough, stage-by-stage tests to evaluate heavy-duty vehicle air brake systems. Typically, your air brake issue can be effectively targeted by one or more of the following standard diagnostic and troubleshooting assessments
DOT CDL 9-Step Air Brake Check
The Department of Transportation (DOT) has standardized a straightforward, 9-step test to safely assess trouble within a truck’s air brake system. The test order is:
Test for static air loss (engine off, parking brake released)
Test for air leaking from the service brake (engine off)
Check low air visual / audible warning system (engine off)
Test and visually assess spring brake valve function (engine on, high idle)
Test air pressure build rate / recovery (engine on)
Test and observe compressor governor cut in and cut-out (engine one)
Test spring brake against parking brake in low gear
Test service brakes, note delayed stop or pulling to one side (engine on)
The benefit of this test is its simplicity in ruling potential issues out, narrowing down the scope of the problem. It can be performed quickly. These characteristics make the DOT assessment similar to the CDL-regulation “Leaks, Alarms, Buttons, Safety Brake” (LABS) test mandated prior to each trip.
The downside of either assessment is their relative lack of precision, compared to other tests. Typically, after narrowing down the issue with the DOT test, it’s wise to continue troubleshooting with a more granular method.
NHTSA Multi-Stage Assessment
One of the most widely-used, precise, thorough evaluations is the standard, multi-stage assessment established by the National Highway Traffic Safety Administration (NHTSA).
It’s both more precise and more comprehensive than the DOT’s nine-step check. To reliably evaluate and pinpoint problems with a truck’s air brake system, the assessment mandates the following tests:
Low Pressure / Pressure Build-Up Test
System Leakage Test
Chuff Test
Pressure Modulator Valve
Traction Control Valve
Air Delivery Service Leakage Test
Manual Parking Brake Operation Assessment
Emergency Braking Test
Dual Circuit System Integrity Assessment
Tractor Protection Valve Check
NHTSA publishes a free guide detailing the methods and sub-steps for each test.
If this assessment doesn’t effectively identify a specific vehicle’s air brake problem, NHTSA also provides a guide to advanced troubleshooting procedures for air brake compressors.
State Department of Motor Vehicles’ Standard Air Brake Inspection
Finally, different states’ DMVs detail how to integrate thorough, comprehensive air brake checks with general-purpose inspections. These guides are particularly useful to ensure compliance with state highway laws.
Often, state air brake inspection standard guides provide inspection checklists, describing how to assess each part of an HDV’s air brake system for misalignment, leaks, or damage.
Data-Driven Fault Detection Diagnostics (FDD)
Data-driven diagnosis offers objective metrics, so it’s less prone to human error than model-based assessment alone. Moreover, FDD’s precision and algorithmic insights empower technicians to identify problems earlier.
In essence, data-driven detection gathers data from sensors. The diagnostic test begins with placing sensors along the brake system, as well as other relevant parts of the vehicle’s machinery. Then, they precisely quantify variables like valve positions, in-line air pressure, chamber pressure, airflow direction, and rotational speed.
Technicians then use the data with sophisticated algorithms to determine pressure gradients, pressure drop rates, and timing of pressure changes. Then, they can apply these rates and ratios to an analysis of the system, detecting anomalies and patterns. These ultimately point to the sources of the trouble (the “faults”) in the air brakes.
Machine Learning & High-Precision Diagnostic Technology
As technology progresses, air brake system maintenance and repair gets better and easier.
High-precision “nano” leak detectors are already changing the game, making the process of finding even the smallest leaks swift and simple.
And, when it comes to data-driven diagnostics, machine learning models are making connections and discovering patterns beyond human capacity. ML has the potential to both streamline diagnostics, and accelerate innovations like onboard, real-time air brake management systems.
Maintain, Repair, and Replace Air Brake System Components
Once the source of an air brake system’s problems is identified, it’s wise to call in an expert to resolve it.
Certain problems can be solved by simply drying, burnishing, or cleaning components. But, others require a more complex solution. At Point Spring & Driveshaft Co., our team of experts offers comprehensive brake services for each part of the system. Services include:
Installation
Repair
Replacement
Alignment
Relining
DOT inspection training
Based on our extensive model knowledge, and using up-to-the-minute data specific to your truck’s system, Point Spring’s technicians can deliver the just-right resolution to your air brake system dilemma.
For some, that’s simply tightening loose components and replacing a few seals. Others might require holistic repair and replacement of your system—including custom fabrication of parts designed to last.
Smart Air Brake Troubleshooting, Expert Solutions At Point Spring & Driveshaft Co.
Air brake systems are vital to heavy-duty vehicles. When it comes to maintaining or repairing yours, don't take chances with guesswork. Instead, troubleshoot with confidence by working with our experts.
To take advantage of Point Spring & Driveshaft Co.’s services, contact us by calling 888-787-7746. Or, fill out the form below, and one of our certified technicians will reach out to you.