The dual-chamber air tank is inconspicuous, but poor maintenance turns it into a ticking time bomb

When it comes to dual-chamber air chambers, many people may still be unfamiliar with this term, but when mentioning one of the components inside the air chamber, the 'diaphragm,' it becomes much more well-known. Structurally, heavy vehicles differ from small vehicles. Due to their heavy weight and strong inertia, they rely on the powerful braking force of pneumatic brakes when parking, resting, or decelerating. Today, let's learn about the details of brake air chambers.

▎Real Case: No Danger, Just a Scare

Today, while chatting with a veteran driver, we discussed this year's transportation market and I also asked about the issue with the air chamber 'cup'. Now, I would like to share this real case with fellow truckers. Master Zheng's home is right by National Highway 319, which provides great convenience for driving. Every time he finishes loading cargo and goes home, he parks the truck at his doorstep, and only backs it out when he sets off to unload.

So here's the problem: the road from the house to National Highway 319 has a certain slope, and every time a vehicle backs out, it requires some braking force. At around 1 a.m. one day, Master Zheng, as usual, started the car—pumped the tires—tapped the tires—checked the undercarriage. Once everything was ready, he began to back out.

After pressing the brake twice, suddenly the brakes softened, and pressing several more times had no effect. In the emergency, Master Zheng pulled the handbrake directly. Fortunately, the vehicle slid a short distance before stopping, with the rear wheels resting on a stone at the edge of the ditch. At that perilous moment, another pointed coal truck was passing in the same direction, but fortunately, the other driver changed lanes to avoid it.

Regarding this issue, when the mechanic arrived, it turned out that the problem was a damaged and leaking brake chamber diaphragm, which caused brake failure. After it was replaced, everything returned to normal. Here, I want to emphasize an important point: the real name of this 'small diaphragm' is a diaphragm, but people are more accustomed to calling it by its nickname. Next, let me introduce the overall structure of the brake chamber.

▎ Types and Principles of Brake Chambers

"Which vehicles need to be equipped with brakes?" Here I want to answer the question from truck drivers. According to the national mandatory standard GB7258-2017, all wheels of automobiles (except three-wheeled vehicles) and trailers (except trailers with a gross mass not exceeding 750 kg) must be equipped with brakes. These are also divided into traditional hydraulic brakes and air brakes, and those with brake chambers belong to the category of air brakes.

Principle: During the braking process of a truck, compressed air pushes the diaphragm (rubber cup) inside the brake chamber, which in turn pushes the push rod through the slack adjuster to rotate the cam by a certain angle. The cam is eccentric, and as it rotates, it causes the brake shoes to spread and rub against the brake drum to generate resistance, achieving the vehicle's braking effect.

The brakes of a truck are divided into service brakes, parking brakes, and auxiliary brakes. What are the differences between them?

Service brakes: Service brakes are divided into single-chamber brake chambers and dual-chamber brake chambers. From the perspective of vehicle design principles, the braking force on the front wheels needs to be less than that on the rear wheels to prevent loss of control during emergency braking. Therefore, the front wheels responsible for steering have only one service brake chamber. This type of brake chamber is a single-chamber brake chamber, which only activates during service braking and has advantages such as simple structure, compact size, and easy maintenance. You can observe a small pack next to the front wheel.

Parking Brake: Most highway trucks are rear-wheel drive, though four-wheel drive vehicles for climbing slopes, such as those used for hauling logs or stones, are also possible. Generally, the axle that handles driving and heavy loads will use a larger dual-chamber air brake, which provides stronger braking force. Structurally, a dual-chamber air brake simply has one more chamber than a single-chamber one. Among the two internal chambers, one is responsible for service braking, while the other is for parking braking. This setup can be operated according to practical scenarios and is commonly seen on drive axles and trailer axles.

The dual-chamber air tank plays a significant role, and its structure is not complicated. It consists of two air ports, connected respectively to the service chamber and the parking chamber. You can see the two small tubes extending upward, which need to be well protected. This is the airway of the air tank, and there must be no air leakage. Afterwards, depending on different needs, different chambers can be inflated or deflated to complete the required operation.

Auxiliary braking: It is called auxiliary braking because it works in conjunction with the brake air chamber. Common examples include exhaust brakes, in-cylinder brakes, and hydraulic retarders, with the hydraulic retarder being the most effective, as it can replace brake spraying. For more details, you can visit http://www.360che.com/news/210611/162731.html, where the author has provided an introduction and comparison.

Key point to note: Here, if everyone confuses driving brakes with parking brakes, it can also be understood that driving brakes are 'air-on brakes,' while parking brakes are 'air-off brakes.' The name 'air-off brake' is quite significant. Through a powerful spring in the brake chamber, during driving, compressed air pushes the spring up, and when the handbrake is applied, the air is released, and the spring locks the brakes. Therefore, low air pressure during driving can also create a braking effect.

Compared with conventional handbrakes, handbrakes achieve parking by locking the drive shaft. Some vehicles are equipped with a mechanical handbrake and air brake structure, which means there is a brake chamber but no air-off brake. When using the foot brake, compressed air also works in the brake chamber, but if the handbrake or drive shaft fails, the parking brake will fail. In contrast, with an air-off brake, if any component such as the air pump, pipeline, air reservoir, or brake valve fails, the driving brake will fail, while the parking brake will still engage to stop the vehicle and avoid risk.

Many overloaded vehicles, after experiencing driving brake failure, achieve deceleration braking by indirectly loosening and tightening the air brake. However, you must never directly lock the air brake, as this can cause the vehicle to lose control. Also, the indirect loosening and tightening of the handbrake should not be done too frequently; in principle, this operation depletes the air quickly, and once the air pressure is exhausted, it becomes ineffective. In emergencies, it can only be used as a temporary measure.

▎What is the failure rate of brake chambers?

As the saying goes: 'Knowing how to drive but not how to maintain the vehicle is useless.' Nowadays, vehicle maintenance requires not only driving skills and sourcing cargo but also understanding the vehicle's maintenance cycle, regularly inspecting and replacing parts, and ensuring stability on the road, which is a primary factor for driving safety. So, what are the key points to pay attention to for this dual-chamber brake that combines service brake and parking brake?

Under normal circumstances, the failure rate of a single-chamber brake chamber is very low, and dual-chamber brake chambers are not that easy to fail either. People often hear about repairing the brake master cylinder, but rarely about repairing the brake wheel cylinder. At most, due to poor spring quality, the brake pedal may not return properly, causing the vehicle to drag during braking.

However, if the energy storage spring inside the parking brake chamber is not functioning properly, it will reduce the vehicle's parking brake capability, preventing the wheels from locking fully and causing the vehicle to roll. This issue is far more serious than a handbrake line air leak. An air leak at most causes loss of air pressure, leading to wheels locking and preventing driving. But if the diaphragm's 'rubber cup' is damaged, it can result in the situation experienced by Master Zheng: weakened service braking and brake failure. If the diaphragm of a dual-chamber brake chamber is damaged, it can also lead to incomplete release of the parking brake, severe wheel dragging, and even tire tread failure.

It should be noted that the energy storage spring inside the brake chamber provides braking force for the parking brake and does not rely on compressed air to function. Therefore, to provide sufficient braking force, the energy storage spring in the brake chamber is extremely powerful. Under no circumstances should you attempt to repair a brake chamber with an energy storage spring yourself. If a problem is found, it must be handled by professional maintenance personnel to avoid incidents of personal injury caused by improper operation.  

Regularly observe the vehicle for air leaks. If the vehicle has just been parked and the air in the air tank leaks away in less than half a day, there is a serious air leak problem that requires thorough inspection of the air system at a repair shop. Do not act carelessly. Also, in the quiet of late night, you can listen to the 'sounds' of the vehicle. Air leaks can produce a faint 'hissing' sound. As a professional truck driver, your sense of hearing is much more sensitive than that of the average person.

In summary, the failure rate of the brake chamber is not high, but it cannot fail. As the saying goes, pulling one hair can affect the whole body; once the brake chamber fails, it becomes a fatal problem. In the current operating conditions of standard-load transportation, driving on highways is more common, so the frequency of braking is also reduced. It is still recommended to test the brakes a couple of times before going down a steep slope. If there is a problem, you can downshift in advance to decelerate.  

▎Extra remarks  

The structure of a truck is so precise that it is astonishing; any part losing its function can lead to the paralysis of the vehicle. Therefore, both while driving and during parking intervals, attention must be paid to every movement of the vehicle. Vehicle failures generally have certain signs beforehand. Detecting them in time and carrying out maintenance can avoid the problem. At the same time, the driver's emergency response is also very important. From Master Zheng's case, we can see that if it is a novice driver, in a state of panic, they would not even think of pulling the handbrake or controlling the vehicle's direction to utilize resistance from objects behind. The saying 'learn from a setback' applies here; experienced drivers gain their knowledge through practice.