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Service Trailer Brake System: What It Is And How It Works

service trailer braking system

When driving at high speeds, stopping a tow truck and trailer that weighs thousands of pounds requires a lot of strength. You cannot rely solely on the braking system of the car you are towing. To stop your trailer properly, you need a complete braking system. The towbar of the towing vehicle may not carry more than 12-15 percent of the weight of the trailer. In this post, we will discuss everything you need to know about the braking system of a service trailer.

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What does it mean to do brake system service?

Semi truck trailers and other large vehicles require a specialized braking system called a service trailer braking system. An air compressor provides the power required to operate this service braking system. Each trailer on a multi-trailer vehicle may have its own air pump to service the system, but in most cases the air is routed from the vehicle to each trailer through an air line.

tow a trailer? Here are some braking options

Overvoltage brakes:

Emergency hydraulic brakes are known as shock brakes. The trailer’s own weight acts as a brake, slowing the vehicle’s progress. When the driver of a vehicle brakes, a hydraulic cylinder is pressurized and the resulting pressure is transferred to the trailer’s brakes, braking the trailer.

Electric trailer brakes:

Electrically coupled trailers have braking systems that utilize electromagnetic brake drums and electrically couple to the towing vehicle. Thereafter, stopping is entirely at the discretion of the driver.

When using an electric trailer braking system it is necessary to have a trailer brake controller installed in the towing vehicle.

How the service braking system works

When using a service braking system, the brake pressure is changed by opening or closing an air valve. It controls the airflow in the cylinder. Trailer brakes are operated by air pressure rather than hydraulic. This is due to the time it takes for the hydraulic fluid to get to each semi truck brake unit. Instead of relying on hydraulic fluid to flow where it’s needed, the technology uses compressed air to instantly change brake pressure.

The service brakes on semi-trailers and trailers are replaceable because they are all connected by the same airline. The cylinder of the main semi truck gets air from the compressor, driven by a drive belt or rotary vane motor. The braking system chamber of each trailer receives compressed air from the master cylinder via airlines.

The trailer brake chamber has a check valve that generates pressure in the system. Each trailer’s 4 wheels have small chambers with pistons that receive air from the master cylinder through the back of the check valves. Air pressure is created, pushing the pistons out of their cylinders and onto a brake lining. When the trailer travels downhill, the pressure exerted on the liner causes friction, causing it to slow down and come to a stop.

When should you service the brakes on a trailer?

Pay attention to the message from the service trailer braking system that it’s time to have your trailer’s brakes checked properly. You should request one of the following to repair your trailer brakes:

  • Those brakes on your trailer don’t work as well as they used to.
  • The brakes make strange noises.
  • It feels like the brakes are mushy or not responding properly.
  • The brake pedal is almost level with the ground.

If you see any of these, it’s time to reset your trailer’s braking system. Waiting too long can seriously damage the brakes.

Disable the trailer service braking system and reset procedures

Disabling the service trailer braking system is easy. Only a handful of actions are required to complete the procedure. But you can only achieve it with the right equipment.


  1. The necessary equipment includes:
  2. Phillips screwdriver
  3. Hammer
  4. A piece of wood or a piece of stone


Finding an assistant to help you with your work is essential. The service trailer braking system can be disabled or reset by following these steps:

1. Locate the trailer adjustment nut

Locating the trailer adjustment nut is a high priority. The normal location is behind the trailer brakes on most vehicles. In most cases you will find this nut near the axle. If you have determined that the adjusting nut is the culprit, continue to the next step.

2. Take the screwdriver and unscrew the lock nut

The trailer brake adjusting nut is held in place by a nut. Here is a lock nut that needs to be loosened with a flat screwdriver. Moving quickly to the next step is possible after loosening this lock nut.

3. Make sure the adjusting nut is turned clockwise

After the locknut has been removed, the adjusting nut can be loosened. Then turn the nut used for fine adjustment of the device clockwise. Keep doing this until it rests snugly against the back plate of the part. Concerns will arise if the nut is overtightened.

The trailer braking system is reset by following these three simple actions. However, if this warning continues to appear on your dashboard, there may be a problem with the trailer brakes.

Estimated costs for trailer brake repair

Depending on the problem with the trailer braking system, you can pay anywhere from zero to five hundred dollars for repairs. You can get by with just a few minutes and a screwdriver if a simple adjustment is needed. However, there are other scenarios that require financial investment.

For example, a replacement fuse can cost between five and fifteen dollars. You can also solve this yourself. Once you locate the fuse, you should be able to complete the job in a few minutes.

If none of these are the case, it may be time to service the trailer braking system or its components. The total amount needed to replace it is based on several variables. The first cost is that original equipment manufacturer (OEM) equipment and components are more expensive. It is also important to consider the type and model of the car as some are much more expensive than others to maintain. It is important to consider not only the cost of the service, but also the location of the provider; for example, metropolitan mechanics will charge much more than their rural counterparts.

Frequently Asked Questions

How to reset the trailer braking system?

The wheels must be removed from the service trailer and an adjustment made for the wheels to reset the service trailer braking system. Attach the wheel adjustment to the brake shoes with a screwdriver. Finally, reset the wheels so you can spin them quickly without hitting any obstacles.

Why does my truck keep saying service trailer braking system?

A blown fuse, faulty brake light switch, or faulty trailer brake control switch is the most likely cause of a service trailer brake system warning. In addition, improper wiring is a common cause of this problem.


It’s easy for anyone without the mechanical aptitude to reset the service trailer braking system at home. After removing the wheel, you can use a screwdriver or a “star wheel” adjuster to readjust the service trailer braking system.

To use this adjustment, squeeze the brakes until you can no longer turn the steering wheel. That’s when you need to step back so the wheel can spin freely again.

Amir Hussain

Amir Hussain is the founder of Freemium World, a geek by nature and a professional Blog writer . I love to write about new technology trends, social media, hacking, blogging and much more.

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