Key Points for Inspecting Air Brake Systems in Winter

As winter approaches and temperatures drop sharply, trucks with air brake systems begin to exhibit typical seasonal failures: air not building up, brakes not returning, and brake failure, causing significant trouble for everyone, and in some cases leading to serious accidents. Therefore, it is very important to conduct a thorough inspection of the vehicle's air brake system before winter. The main air brake systems used in domestic trucks have not changed much in their basic structure despite improvements and upgrades over the years, but new components such as ABS sensors and air dryers impose higher demands on the braking system, which also require special attention.

 Through surveys of truck drivers and investigations of some accident cases, it has been found that some serious accidents, such as trailer fires and brake failures, are related to the braking system. Often, small malfunctions or component issues eventually lead to major disasters. Some faults occur because the system is not regularly checked or counterfeit parts are used, and when these parts appear to be in good condition at the time, they can easily create a false sense of security. The following outlines a few key points regarding inspections and regular replacement of parts.

● Air compressor inspection: Severe oil leakage can lead to downstream failures

When routinely inspecting the relief valve exhaust port, pay attention to whether there is severe oil contamination. If the oil contamination is serious, the air compressor needs maintenance and replacement if necessary.

 Pay special attention to whether the air compressor oil lubrication pipe is intact. Be aware that the oil pipe should not be repaired by welding when installed, and check for any abnormal forces or vibrations during installation. A broken lubrication oil pipe can easily cause serious damage to the air pump and engine. The vehicle should carry a spare of this oil pipe for replacement when needed. Check whether the air compressor intake pipe is intact to ensure clean and sealed air intake. (The type of air compressor varies with different engines.)

During cold starts in winter, it is strictly forbidden to operate the engine at high throttle before the water temperature reaches 60-80°C to avoid cylinder scoring and excessive wear due to poor lubrication of the air compressor, which can cause oil leakage and carbon buildup in the cylinder head (air compressor).

● Check the dryer; high intake air temperature affects regeneration performance.

   It is difficult to judge the performance of a dryer based on its appearance. Since a dryer completes a work cycle, it requires two actions: 'absorbing water and discharging water.' In addition to choosing a high-quality dryer, it is necessary to check whether the system is operating normally. For example, if there is serious oil contamination at the exhaust port, it indicates that the dryer's molecular sieve has been heavily contaminated by oil and has completely failed, no longer possessing the ability to absorb water.

   Check the back-blow performance of the dryer. When a properly functioning dryer undergoes pressure release, compressed air from the back-blow tank or air reservoir will back-blow through the dryer’s molecular sieve, expelling moisture. At this time, there will be a 'puff' sound and water vapor will be discharged from the exhaust port. If there is no exhaust action, the dryer base, pressure relief valve, four-circuit protection valve, and other components should be inspected and replaced if necessary.

   The intake air temperature should be checked, which can be done using temperature test paper. When it exceeds 65°C, the performance of the dryer filter decreases or may even fail. In particular, for certain batches of Dongfeng Tianlong series tractors, measures such as installing a spiral tube are needed to achieve cooling. In winter, snow splashed by tires can easily cause electrical circuits to freeze, leading to short circuits and wire breaks. Circuits should be wrapped with plastic wrap, and a small amount of grease or lubricating oil should be applied to prevent moisture condensation.

    The air pressure sensor of the air reservoir and the ABS solenoid valve are also wrapped in cling film to prevent signal failure and false alarms caused by icing. This is a common trick used by bus drivers in the north. 

● Air pipeline inspection: Pay special attention to cracks in movable air pipes.

   ‘Flexible’ air hoses refer to a type of airway that undergoes movement, such as the brake caliper hoses connected to both ends of the vehicle frame, or the spiral air hoses between a trailer and the tractor. These hoses are made of special materials and are prone to fatigue, cracking, or even breaking after long-term movement.

 The 'flexible' air hoses cannot be replaced with hydraulic pressure hoses. This is because hydraulic hoses have a steel wire lining and allow only a certain range of bending deformation. For example, in the front and rear axle master-slave cylinder lines, where there is significant extension and retraction movement, using hydraulic hoses interchangeably can easily cause internal cracking. High-quality air hoses should be rubber hoses specifically for air, with an external steel wire protective layer and an inner fabric-reinforced layer. The external steel wire layer helps distribute external deformation forces and prevents abrasion and interference.

   Pay special attention to the connection parts of the trailer air pipes, especially the 'substandard large pipes' made of low-quality PVC material, which are often mixed. They are extremely brittle in low temperatures and prone to cracking and breaking.

    If low-quality 'Yada pipes' are installed between the brake chambers and the emergency relay valve, air leakage, if not detected in time, can cause brake failure. Especially if there is a crack and air leakage in the parking brake line, the spring action can cause the brakes to grab, leading to overheated brake hubs. According to statistics, 70% of semi-trailer fires are caused by air circuit failures; this is not an exaggeration.

    Pay special attention to whether there is any air leakage at the sub-cylinder air pipe interface. The parking brake line inspection needs to be carried out with the brake released. Ensure proper vehicle anti-roll measures are in place and, under safe conditions, inspect each part one by one. Do not allow air leaks in the pipes to cause poor brake return, which could result in brake drum overheating and lead to fire. ● Brake Return Inspection: Pay attention to active return and passive return.

① Active Return Part

Brake return actions are divided into active and passive returns. Passive return specifically refers to the return springs of the brake shoes inside the brake drum, the shoe retaining pins, and similar mechanisms. Active return actions are influenced by multiple components. First, with sufficient compressed air, press down the brake pedal and quickly release it, observing the exhaust condition of the relay valve or quick-release valve. If the exhaust is slow, timely maintenance and replacement are required. Most valve body failures are related to oil contamination and water ingress, so pay attention to checking the dryer.

Pay attention to check whether the brake caliper return spring is broken or fatigued; never underestimate this small spring. According to the lever principle, the force of the return spring will be greater than that of the passive return spring on the brake shoes. If necessary, replace it with a high-strength return spring to improve the response time of the braking action and ensure reliable return. In addition, this small spring ensures good brake return, which can save more fuel and thoroughly improve brake grabbing.

② Passive return part

    The passive return components refer to various parts inside the brake drum. The return springs of the brake shoes need to be checked by levering them up and down. If they lack elasticity or are stretched due to fatigue, they need to be replaced promptly to prevent the lower brake shoe from returning slowly over a long period, which could cause interference and friction with the brake drum, leading to high temperatures and giving a false impression of tightness when adjusting the brake shoe clearance, making proper adjustment impossible.

   The camshaft inside the brake drum and the brake shoe mounting pins need to be checked for smooth movement. If they are stiff, they should be disassembled, polished, and rust removed, and replaced if necessary. If there is too much play, the brake shoe bushings or the entire assembly need to be replaced.

● Brake pad and hub inspection: Pay attention to the brake hub's wear life when inspecting the brake pads.

    Many truck users like to use 'Brake King.' This kind of illegal increase in braking air pressure is like trying to cure thirst with poison. They often overlook the quality of the brake pads, which further affects braking performance. For example, counterfeit BPW axle brake pads achieve less than 40% of the friction of genuine factory pads, causing brake overheating and premature failure of the brake drums.

The riveting of brake pads requires specialized equipment. Manual riveting or using bolts for fixation can easily become loose. Pay attention to the fact that using a brake king can easily lead to serious accidents such as brake pad breakage or loosening. This is also a major cause of brake hub fires, which is often overlooked.

 After long-term use of the brake hub, the edges may develop a 'mossy' texture. This is a normal phenomenon. As long as there are no cracks in the main body of the brake hub and the interior is smooth, it can be repaired and used normally within a 3mm range. The excess grooves can be ground flat with a grinder, and replacing with thicker brake pads allows continued use, saving on costs.

    Note that vehicles equipped with 'Brake King' cannot be used, as the uneven braking force causes the inner surface of the hub to become irregular, leading to the brake pads and hub being cracked or shattered.

  After the brake drum inner diameter wears larger, using the method of replacing it with a 'camshaft with larger rollers' to restore braking power requires attention. After replacing with larger rollers, the brake action curve also changes, which can easily cause poor return movement, especially when the brake pad clearance is too large. Normally, the brake clearance needs timely adjustment, as the brake shoe return spring has limited effect, and the caliper return spring needs to be reinforced.

● Editor's Note

Brake system inspection is especially important before winter, involving components like brake masters, substandard brake hoses, and dryers, which can be quite harmful. You can't be lazy in daily maintenance, and shouldn't rely entirely on mechanics. As the saying goes, 'Good cars are 70% maintained and 30% repaired.' You should be aware of your own car's issues. Modern 6-axle semi-trailers have 12 braking units, which require extra careful inspection. This is no longer an era of single vehicles; when operating an 18-wheeler, you must not be careless. Advanced theory isn’t necessary; only essential knowledge is needed. The meticulous and careful spirit of the older generation is still useful—keep that in mind.