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2026
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Common problems with trailer braking systems and how to fix them
With the rapid development of road transport, semi-trailer trucks are getting a lot of attention in the industry for their high load capacity, economy and efficiency, gradually becoming the main vehicles for long-distance logistics.
Many drivers who are handling semi-trailers for the first time lack understanding of the structure, design, and maintenance of semi-trailers, and often find themselves helpless when brake system failures occur on the road, sometimes leading to serious accidents. Apart from mechanical issues with the axles and adjustment arms, many faults are caused by the brake air circuits. Today, let's take a look at common faults in the semi-trailer brake system air circuits and their solutions: ● Composition of the trailer brake system
Trailer brakes are generally air brakes, with the air supply coming from the tractor. First, let's take a look at a simple diagram of the trailer's air system. As shown in the diagram, the air system of a semi-trailer isn't complicated, mainly consisting of three key components: the emergency relay valve, the trailer ABS, and the brake air chamber. Most faults come from these three main parts, as long as there are no leaks due to poor sealing. Of course, the issues are comprehensive, and all components related to air brakes can have varying degrees of impact, so installation should not be taken lightly. ● Air system installation requirements
Apart from the three big components, it's mainly the installation of the air system. It seems basic, but if it's not done properly, the performance of the vehicle will be disappointing.
Airline connections should not be excessively long or unnecessarily tangled; a proper airline is usually straightforward.
Also, the inner diameter of air hoses and fittings must meet the standard; if not, braking performance may be poor. An airline done according to requirements not only provides excellent braking performance and lower failure rates, but also makes maintenance and after-sales service easier, hitting several targets at once.
● Common problems and how to deal with them
After understanding the structure of the braking system, let's look at common faults that actually occur. Semi-trailer brake failures generally appear as: air leaks, dragging brakes, and lock-ups! Let's analyse them one by one. Fault phenomenon one: air pressure is constantly below 0.7MPa or obvious dragging occurs while the trailer is moving. How to deal with it: 1. Replace the truck head to rule out tractor faults. 2. Check for air leaks using soapy water to eliminate possible faults such as 'air leak at the handshake connection', 'air leak at pipe joints', 'brake chamber air crossover', 'emergency relay valve air leak', and 'trailer ABS valve air leak'.
If the air leak still exists, check whether the valve body has water ingress or visible rust. This indicates that the trailer's air line filters and drying are not effective, and the valve body may need to be replaced.
Fault phenomenon two: The trailer cannot start after parking and remains stuck.
Solution:
1. First, unplug the ABS spiral cable to see if it starts normally, to rule out electrical issues.
2. Put the tractor in air supply mode, disconnect the air hose from the hand control valve, and check if the air pressure at the 'outlet' is normal (the air pressure should be high when in air supply mode). 3. Check if the parking brake is released. 4. If the air circuit is fine, finally check for mechanical faults in the axle.
The braking system of trailers keeps all braking states and ensures the air supply lines remain unobstructed for safety. The tractor's foot brake and parking brake work by supplying air to the control lines. More and more customers are modifying their trailer air systems themselves. Improper modifications can damage the original braking system, and combined with incorrect use, this can lead to many safety incidents. Vehicle modifications should always be done at authorised modification centres or the original factory.
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