07
2026
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04
The tires lock up with just a light press on the brakes. Where exactly is the problem?
A few days ago, a truck driver with an Auman tractor contacted me, reporting that the brakes of the vehicle were getting worse and worse.
The specific situation is: every time the brakes are applied, even with a light press of the foot, the trailer locks up, accompanied by the smell of rubber from tire wear; after releasing the brakes, the vehicle cannot glide smoothly and promptly, but instead takes several minutes of effortful movement to return to normal. As a result, now whenever braking is required, the driver’s legs start trembling.
After careful inquiry, it was understood that the car's brakes have always been functioning normally. This phenomenon appeared about more than a month ago. The vehicle was taken to a brake shop for repair once, and the mechanic said that the tractor's trailer valve was advanced too much and replaced the trailer valve, but the fault remained and was getting worse and worse.
▎After the inquiry, I got on the vehicle to check
It was observed that the trailer tires were indeed abnormally worn, and the wear was very severe.
When visually inspecting the trailer brake relay valve, the fault was already very clear: the exhaust port of the valve had thick, fresh yellow rust.
Disassembled the trailer relay valve, and it was clearly heard that there was a slight air leak from the exhaust port of the valve. When the air pipe at the control port of this valve was just removed, a stream of water spurted out from the air pipe, and water also overflowed from the control port. When disassembling the valve, a lot of accumulated water flowed out from the control chamber.
Further disassembly revealed that the intake filter of the valve was almost completely clogged with debris.
Due to a slight air leak at the exhaust port of this valve, the exhaust port snap ring was removed to further check the sealing valve. After cleaning a small amount of oxidized particles at the sealing valve port, it was checked that the sealing contact surface of this valve is flat, smooth, and undamaged, and does not affect continued use.
Clean the impurities inside and outside the valve cavity and control cavity, and apply a small amount of special grease to various moving parts and O-rings (note: absolutely do not use wheel-bearing grease, as that type of grease is too thick, slows down movement, and may even cause sticking, affecting performance). After reinstalling and test-driving, the driver said it felt like the old times!
Based on the vehicle usage and the driver’s feedback, the desiccant that had been in use for over a year was also replaced (the desiccant must not be a random or low-quality product), the brake master cylinder was serviced, and water was drained from all the air reservoirs, which greatly satisfied the driver.
▎Conclusion
Actually, every fault symptom corresponds to one or more causes, so when a fault appears, truck drivers should not repair blindly.
Take this truck, for example. When the control cavity of the brake valve is filled with water, pressing the brake does not compress the liquid like air; the liquid instantly pushes the piston down, opening the sealing valve, and the sub-pumps start working. Without the cushioning of compressed air, what we call the “curve feeling” disappears.
Moreover, the control lines are quite long. Even if the brake is released, the water cannot be expelled, and it accumulates more and more, making seizing and dragging worse.
The almost blocked air intake prevents the air reservoir pressure from replenishing in time after braking. Low air pressure cannot push open the stiff springs of the dual large sub-pumps on the trailer, so the released brakes cannot reset immediately. A few minutes later, as the air pressure rises, the brakes automatically return.
Additionally, fine particles produced by oxidation get stuck on the originally sealed valve surfaces, causing the valves not to close tightly and resulting in air leakage (this leakage phenomenon often occurs on various pumps and valves and does not require replacement, just maintenance).
Transport is tough, driving trucks is hard, so cherish each journey.
May fellow drivers uphold their principles and repair rather than replace whenever possible.
Behind the Scenes: This Manuscript — Ka Family Number, We Are Serious.
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