Brake Inspection Checklist

Christian Spaccarotella
6 Minutes


Frequently checking your vehicle will always do you good, but unfortunately it’s not always held up to standard. One of the leading causes of accidents and DOT violations is improper brake maintenance, and that’s why it is critical to inspect your vehicle’s brakes frequently. Brakes follow a strict standard for inspections, where each system has to meet certain requirements set by the Department of Transportation (DOT). To be able to perform a proper brake inspection, knowing each piece to the system is a good starting point.

Brake Systems

There are generally 5 major components to a truck’s air brakes system. Most people who operate the trucks and trailers have a general understanding of these brakes, but sometimes their knowledge isn’t enough to fully understand the true mechanics and how important proper maintenance is to keep themselves and others safe. Here are the basic parts to a braking system.

Air Compressor

The Air Compressor in the brake system “maintains the proper level of air pressure so that the air brakes…operate safely and consistently” (MPC). Different types of trucks have different air compressors, them being either gear or belt-driven, and are cooled by air or an engine cooling system. Every time the engine is started, the air compressor device loads and unloads air. That air is pumped in and out of the two-cylinder compressors and the reservoirs (air tanks).

Common issues with air compressors are air leaks, filter clogs and inadequate lubrication. These are all warning signs that the system is gradually failing. Essentially when performing a brake inspection, you are checking for any audible air leaks, and that the air compressor is holding pressure, typically around 125-135psi. To do this, most of the time people do what is called the “Brake Pedal Hold Test”, where you hold down the brake for 60 seconds. If air pressure drops more than 5 psi, that means you have an air leak and it would be a DOT violation.

Reservoirs/Air Tanks

In heavy trucks, the tanks hold the compressed air until you need to brake. Reservoirs are “pressure-rated tanks that feature special drain valves called draincocks” (MPC). These drain themselves of any moisture or pollutants that might alter the air. Over time, air tanks can build up condensation, especially if transporting from coast to coast. Condensation build up leads to rust and corrosion, which creates weak spots in the metal. LGT and other companies require drivers drain the air system for trucks and trailers everyday to avoid any condensation build up.

Foot Valve

Also known as the treadle or the brake pedal, the foot valve sets the volume of air pressure used. It is set based on how hard the operator presses their foot on the foot valve. When all that air is released, it will take time to produce air through the compressor. Testing the foot valve is also how you would test the air compressor, by holding down the pedal. Problems with foot valves are not frequent, but still a possibility.

Brake Chambers

Brake chambers are what turn the compressed air into mechanical force. It’s the part that triggers the brakes and actually makes the vehicle stop. The chamber is held together by a clamp assembly “that is specially made to regulate the compressed air that is released into the chambers” (MPC). The amount of brake chambers per vehicle is determined by the number of drive axles. There is 1 brake chamber per side per axle, so with trucks and trailers like LGT’s, there are 6 brake chambers to check. Each of the chambers has a specific pushrod stroke adjustment limit. When the brake push rod stroke exceeds that limit, the brake is out of adjustment. Brake chambers are reliable, but often the most common part to fail. When the diaphragm internal part wears out or gets pinholes, it’s time to replace it. Brakes could fail if there are leaks or holes that allow too much air to be released too quickly. The slack adjustors are automatic rather than manual in more recent years.

Brake chambers are very dangerous to work on because the spring inside the chamber is compressed under very high tension. Tampering, corrosion, and/or damage can cause the spring to release, which causes a very violent motion of parts in the chamber leading to serious physical harm. Typically to get these fixed, you must take it to a specialized or certified shop to repair.

Brake Shoes and Drums


Brake Shoes are the stopping force for the brakes on both drum and disc applications. DOT requirements place the minimum threshold of drum brakes at ¼” or 8/32”. Disc brakes have a DOT threshold of 1/8” or 4/32”. LGT measures the thickness in 32nds of an inch.

Brake Drum lining/Rotor Condition

The brake drum and rotor assemblies are the parts in which the stopping force is applied to slow down the vehicle. These items must also be checked on a regular basis.

It is common for brake drums to develop heat cracks due to high frictional temperatures, especially when the brake shoes are worn down too much. Brake shoes are riveted onto the hardware in drum brake shoes and when the rivets are exposed, they will wear a groove in the metal drum.

Rotors paired with disc brakes are better at dissipating heat, however can still be grooved and/or warped through normal wear and tear.

Drum vs Disc

Drum vs Disc brakes has been a common question in the transportation industry well up to this point in time in the 21stcentury. Most of LGT’s fleet is equipped with drum brakes, specifically our trailers. During the second half of the 20thcentury, the transportation industry started to consider disc vs drum brakes. However, Disc brakes that could stop a transport vehicle hauling up to 80,000 lbs were inherently bulky and added too much weight to be economical for the end user. This has finally changed due to engineering upgrades to the caliper and rotor design making them weight equivalent to drum brakes. Disc brakes will beat drum brakes in nearly every category:

  • Maintenance Cost
  • Maintenance Life
  • Performance/Stopping Distance
  • Heat Dissipation

There are still a few cons to disc brakes, but they are heavily outweighed by the pros listed above.

Brake Inspection Overall

Overall, to perform a brake inspection, you must visually inspect everything. In order to know that the brakes are properly functioning, you have to witness them working. Measuring can be done with tests, like the brake pedal test, but you have to actually see and listen for leaks or weak spots. Some things to double check are:

  • brake shoes: see if they are out of adjustment
  • automatic or manual slack adjustors
  • air compressor/air tank leaks
  • rust or corrosion on any part of the system
  • seals and valves: check for holes or leaks
  • is the air compressor holding pressure at the specified range

Useful tools/measurement techniques – LGT

Here is a list of some of LGT’s brake system suppliers for reference:

  • Bendix – tractors (Paccar units)
  • Wabco – tractors (Freightliner units)
  • Wabco – trailers (ABS with Roll stability)

Here are some measurement tools to measure brakes:

  • Tire Tread Depth Gauge – made to measure tires, but can measure brake shoe thickness as well in 32nds of an inch.
  • The  tool below is specified for measuring brake drum shoes while equipped on the vehicle
  • This is the Bendix Disc Brake Pad measuring tool that we use to measure disc brakes

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