Brake System Analysis 1: Is Upgrading to Big Brakes Reliable?

Brake King is a well-known product in the heavy truck community, and many truck drivers are familiar with it. Simply put, it is a relay valve that increases the air volume and ultimately transmits force to the brake drum through the sub-cylinder air chamber. Many users who have tried it say it works very well, making the brakes very powerful, which is especially beneficial for trucks frequently driving on mountain roads or heavy downhill loads. However, some truck drivers believe that modifying their trucks with Brake King can damage other components of the braking system and may even lead to accidents. So, is installing Brake King really reliable?

The quality of a vehicle's braking performance is absolutely not something to be ignored. An experienced driver once said, 'If you want to learn how to drive, the first thing you need to learn is how to brake.' This is especially true for heavily loaded trucks, as the braking system can be directly related to the driver's life safety. Whether it's upgrading to high-performance brakes or installing an ABS braking system, the original intention is to reduce the occurrence of accidents.

The forum truck enthusiast 'Yonghong', whom we usually call Brother Hu, is an experienced driver with many years of experience in brake system maintenance. This time, Truck Home made a long journey to Changli to visit the repair shop operated by Brother Hu and seek his insights on brake systems. 

● Brake Test Structure and Principles

To make brake testing more intuitive and the data quantifiable, Brother Hu built two simulated test rigs, one for the tractor and one for the trailer. Both test rigs are equipped with ABS devices. In this article, I will focus on the tractor brake system. This simulated tractor brake system test rig has a total of four axles, which can accommodate the testing of the two mainstream tractor brake systems on the market: 6×2 and 6×4. The simulated wheelbase is identical to that of the real vehicle, and it includes all necessary controls and a brake pedal.

The front half of the test bench is for the steering brake of the 6×2 vehicle type. The braking system in this part consists of a main control valve, a relay valve for the steering brake, and two ABS solenoid valves. It uses a control method with left and right air circuits, meaning that the left and right sides of the first and second steering axles are controlled separately, as the braking of the tractor's steering wheels is side-controlled. Regarding the layout position of the ABS sensors for the steering wheels of 6×2 vehicles, it varies among major manufacturers. For example, Foton places the sensors on the first axle of the tractor, while Sinotruk arranges them on the second axle.

Based on many years of maintenance experience, the layout position of the sensors does vary. Affected by factors such as axle load, Hu personally believes that for 6×2 models, the sensor arrangement on the steering axle, the second axle is better than the first axle.

Regarding the rear drive axle section, we can see that it is composed of the tractor's air reservoirs, the trailer valve, the rear axle relay valve, and the parking brake relay valve, among others. Here, we focus on the trailer valve, which is used to convert the control of the trailer brakes from the master pump and handbrake signals. The rear axle relay valve is used to control the braking of the drive wheels.

Through this test bench, it is easy to understand that the principle of the tractor braking system is relatively simple, mainly controlled by a master cylinder and various relay valves. ● Blind modifications affect braking

As vehicles accumulate mileage during transportation, the original braking system will develop problems to varying degrees. These issues are mainly reflected in insufficient braking force. When encountering such situations, many drivers, as described earlier, choose to replace the original trailer valve with a 'Brake King'.

Regarding this practice, Hu believes that many drivers modify their Brakes Kings to increase air volume due to their personal driving habits. After Brakes King increases the air volume, the brake response time of the trailer feels very short, and the braking effect is better.

But in fact, this approach has its pitfalls. According to his experience, the tractor and trailer brakes must work in coordination to ensure effective braking. This is because the brake master cylinder controls the front axle relay valve, rear axle relay valve, and trailer valve simultaneously. Increasing the air flow to the trailer valve alone will undoubtedly disrupt the coordination of the entire system. Many drivers do not realize that choosing to modify the brake booster often leads to frequent mismatches between the tractor and trailer brakes.

Brake King Technology Principle Diagram

If the Brake King is replaced blindly, it will speed up the response time of the trailer brakes. The trailer brakes will engage too early, and during normal driving braking, lightly pressing the pedal feels like the braking effect is fine. However, if you press hard or quickly — what we commonly call emergency braking — the response time of the tractor's brakes will be slower than the trailer's, making it feel like the tractor has no brakes.

Many drivers who have upgraded to high-performance brakes or added a relay valve to the trailer valve aim to speed up the trailer's brake response time, but this often disrupts the vehicle's overall linear braking performance. In such cases, Hu usually restores the trailer valve to its original state to reduce incidents of wheel lock caused by not properly controlling the brake pedal force. It's similar to riding a motorcycle—if an emergency stop is needed, both front and rear wheels should be used, rather than just the rear brake. Another common issue in braking systems is slow release of the handbrake. According to Hu, if the trailer's handbrake releases slowly but the foot pedal brake works normally, the problem is likely related to the air supply to the tractor's handbrake. National regulations require that a one-way valve be installed in the handbrake air supply. Its function is to ensure that, if certain air lines in the tractor break, the air pressure in the handbrake and the handbrake relay valve remains effective, preventing emergencies where the tractor could lose control.

On the other hand, the handbrake returns slowly, which in most cases is related to the spring strength in the one-way valve. If the spring is too strong, it will affect the air flow. We also tested the handbrake return time on-site by connecting the test benches of the tractor and trailer. The handbrake took about 7 seconds to return when released, mainly due to the one-way valve supplying air to the tractor's handbrake.

二控三布置形式

● Editor's Note:

Through this visit to truck enthusiast Brother Hu's brake system test bench, I believe everyone has gained a clear understanding of the structure and principles of the tractor's components. In order to achieve better braking performance, whether or not to install the Brake King was also explained by Brother Hu from a technical standpoint. However, some truck enthusiasts stated that in situations where they carry heavy loads and drive on mountain roads, they would rather risk damaging the brake drums than not install the Brake King to ensure the truck can stop.

Regarding driving safety, it is recommended:

First, try not to overload, as the consequences are unpredictable.

Second, regularly maintain other components of the braking system, and never overlook small details that could cause major damage.

Additionally, current regulations do not require commercial vehicles to be equipped with an ABS system. It is hoped that truck drivers take responsibility for themselves and choose to install ABS to assist braking. Compared to the importance of life safety, the cost of installing ABS is truly negligible.

We will conclude our introduction to the structure of tractor braking systems and common issues here. In the next session, we will provide a detailed explanation of the trailer braking systems we observed during this visit.