The dual-chamber airbag doesn’t look like much, but poor maintenance can turn it into a ticking time bomb

When it comes to dual-chamber brake chambers, many people are probably still unfamiliar with the term. But when you mention one of the components inside the chamber—the 'diaphragm'—it’s much more well-known. Structurally, heavy-duty vehicles differ from smaller vehicles. Because of their weight and strong inertia, they rely on the powerful braking force of pneumatic brakes for stopping and slowing down. Today, let’s take a look at some things about brake chambers.

▎Real Case: A Narrow Escape

Today, while chatting with an experienced driver, we talked about this year’s transportation market and I also asked about the air chamber 'rubber bowl' issue. Now I’m sharing this real case with fellow drivers. Master Zheng’s home is right by National Highway 319, which makes driving very convenient. Every time he finishes loading goods and comes home, he just parks the truck at his doorstep and only backs it out when it’s time to head out for delivery.

So here's the problem: the road from the house to National Highway 319 has a certain slope, so every time he backs the car out, it needs some braking force. Around 1 a.m. one day, Master Zheng, like he usually does, started the car, pumped the tires, tapped the wheels, and checked the chassis. Once everything was ready, he began backing out.  

After pressing the brake twice, suddenly the brakes felt soft. A few more presses did nothing. In the critical moment, Master Zheng pulled the handbrake. Luckily, the car slid only a short distance before stopping, with the rear wheels hitting a rock by the edge of a ditch. It was a close call. At that time, another coal truck was driving in the same direction, but fortunately, the other driver changed lanes to avoid a crash.

Regarding this issue, when the mechanic came, it turned out that the problem was a damaged and leaking brake chamber diaphragm, which caused the brake failure. After replacement, everything returned to normal. Here, I want to highlight something for everyone: the real name of this 'diaphragm cup' is actually the diaphragm itself, but people are more used to calling it by its nickname. Next, I’ll introduce the overall structure of the brake chamber.  

▎Types and Principles of Brake Chambers  

"Which vehicles need to have brakes?" Let’s answer a question from drivers here. According to the national mandatory standard GB7258-2017, all wheels of vehicles (excluding three-wheelers), and trailers (excluding trailers with a gross weight not exceeding 750kg) must be equipped with brakes. These are further divided into traditional hydraulic brakes and air brakes, and the ones with brake chambers fall into the air brake category.

Principle: When a truck brakes, compressed air pushes the diaphragm (or cup) inside the brake chamber, which then moves the push rod through the adjusting arm 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, creating resistance and making the vehicle slow down.

Truck brakes are divided into service brakes, parking brakes, and auxiliary brakes. So, what’s the difference between them?  

Service brakes: Service brakes are divided into single-chamber brake chambers and double-chamber brake chambers. From a design perspective, the braking force on the front wheels needs to be less than the rear wheels to avoid losing control during emergency braking. So, the front wheels, which handle steering, only have one service brake chamber. This type of chamber is also called a single-chamber brake, which only activates during regular braking. It has advantages like a simple structure, small size, and easy maintenance. You can even spot a small lump next to the front wheels.

Parking Brake: Most regular highway trucks are rear-wheel drive, though there are four-wheel drive vehicles for climbing mountains, like when hauling logs or stones. The axles responsible for driving and carrying heavy loads usually use bigger double-chamber air brakes, which provide stronger braking power. A double-chamber air brake is basically a single-chamber air brake with an extra chamber inside: one chamber handles driving brakes, and the other handles parking brakes. You can use them depending on the real-life situation, and they’re commonly found on drive axles and trailer axles.

The dual-chamber air tank really plays an important role, and its structure isn’t complicated. It has two air ports that connect to the service chamber and the parking chamber, respectively. You can see the two small tubes coming out at the top, so you should make sure to protect them well—these are the air passages of the tank, and there must be no leaks. Later, depending on different needs, you can inflate or deflate the different chambers to complete the required operations.

Auxiliary braking: It's called auxiliary braking because it works together with the brake air tank. Usually, we see it in exhaust brakes, engine brakes, and hydraulic retarders, with the hydraulic retarder being the most effective. It can even achieve the effect of replacing brake water spraying.

Key point to note: Here, if people confuse driving brakes with parking brakes, it can be understood as the driving brake being an 'air-on brake,' while the parking brake is an 'air-off brake.' The name 'air-off brake' isn’t small—through a powerful spring inside the brake chamber, the compressed air pushes the spring during driving. When you pull the handbrake, releasing the air lets the spring lock the brake. So, if air pressure is too low while driving, it can also create a braking effect.

Compared to a regular handbrake, where the handbrake works by locking the driveshaft, some vehicles have a mechanical handbrake combined with an air brake structure. In such cases, there’s a brake chamber but no air-off brake. The foot brake still works by compressed air entering the brake chamber. However, if the handbrake or driveshaft fails, the parking brake won’t work. On the other hand, with an air-off brake, if any component like the air pump, pipe, air tank, or brake valve fails, the driving brake will fail, but the parking brake will still engage, allowing the vehicle to stop and avoid risk.

Many overloaded vehicles, when the service brakes fail, achieve deceleration by indirectly loosening and tightening the air brakes, but you should never directly lock the air brakes, as this can cause the vehicle to lose control. Also, the indirect loosening and tightening of the handbrake should not be done too frequently, because in principle, this method releases air very quickly, and once the air pressure is gone, it becomes ineffective. It can only be used as an emergency measure in urgent situations.  

▎How common are brake chamber failures?  

As the saying goes, "You can drive, but if you don't maintain your vehicle, it's all for nothing." Nowadays, maintaining a vehicle isn’t just about being able to drive or finding cargo—you also need to understand the maintenance cycles of the vehicle, regularly check and replace parts, and ensure stability on the road. This is a key factor for driving safety. So, for the dual-chamber air brakes that combine the service brake and parking brake, what are the key points to pay attention to?

Normally, single-chamber air springs have a very low failure rate, and even double-chamber ones aren't that easy to break. Most people have heard of repairing brake master cylinders, but rarely of fixing brake wheel cylinders. At most, a bad spring can make the brake pedal not return, causing the vehicle to drag when braking.  

But if the parking brake chamber's energy storage spring fails, it can reduce the vehicle's parking ability, making it impossible to lock the wheels and causing the vehicle to roll. This is way worse than a leaking handbrake line. A leak usually just causes a loss of air pressure, making the wheels lock up and the vehicle unable to move. But if the diaphragm gets damaged, like the 'rubber bowl,' it can cause what Master Zheng experienced: weaker service brakes and brake failure. If the diaphragm in a double-chamber air spring is damaged, the parking brake may not release fully, the wheels could drag badly, and the tires could even be ruined.

It's important to note that the return spring inside the brake chamber provides braking power for the parking brake and does not rely on compressed air. Because of this, the return spring inside the brake chamber is extremely powerful to ensure enough braking force. Never try to repair a brake chamber with a return spring by yourself. If there's a problem, wait for a professional mechanic to handle it to avoid any accidents caused by improper handling.

Regularly check your vehicle for air leaks. If your vehicle just stopped and the air in the tank runs out in less than half a day, that means there's a serious leak, and the air system needs to be inspected thoroughly at a repair shop. Don't take it lightly. Also, in the quiet of the night, pay attention to the sounds your vehicle makes. Air leaks often produce a faint hissing sound. As a professional truck driver, your sense of hearing is usually sharper than the average person, so use it to your advantage.

In short, the failure rate of the brake chamber isn't high, but it can't fail either. As the saying goes, 'a single hair can affect the entire body.' Once the brake chamber malfunctions, it becomes a serious problem. In current operations with standard-load transportation, most driving happens on highways, so the brakes aren't used very often. It's still recommended to test the brakes a couple of times before going down a steep slope, so if there's any issue, you can downshift in advance to slow down.  

▎Extra notes  

The structure of a truck is astonishingly precise; if any single part fails, it can cripple the whole vehicle. So while driving or even when parked, you need to pay attention to every little thing the vehicle does. Vehicle malfunctions usually show some signs first, and noticing them promptly and getting repairs done can prevent bigger problems. At the same time, a driver's emergency response is super important. From Master Zheng's case, we can see that if you're a newbie, panicking means you won't even think to pull the handbrake or steer the vehicle to find resistance from objects behind you. The saying 'learn from mistakes' really applies here—experienced drivers get there through practice.