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2025
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What causes the brake to return slowly after releasing the brake pedal?
Some netizens reported that they drive trucks and recently have felt that after releasing the brake pedal, the brakes return very slowly. They asked what causes this. In fact, this phenomenon is technically called "brake drag." Brake drag refers to the situation where, after the driver fully releases the brake pedal, the vehicle's brakes are not immediately released or cannot be completely released, and there is still a considerable amount of braking force.
During the braking process of a car, the final stage is called the 'brake release time,' which generally lasts 0.2 to 1.0 seconds. If, after the driver releases the brake pedal, the car continues to decelerate beyond this period, it indicates a phenomenon known as brake lag. Brake lag is a type of brake system malfunction in vehicles, which can lead to increased fuel consumption, accelerated wear of the brakes, overheating of the brake components, and other issues. Brake lag usually occurs in trucks that use air brake systems, while it is relatively rare in passenger cars that use hydraulic brakes. The causes of vehicle brake lag can generally be divided into two main categories: the air pressure control system and the wheel brakes. A fault in the air pressure control system can cause brake lag in the entire vehicle, whereas a fault in individual wheel brakes can lead to brake lag in specific wheels.
▎1. Air Pressure Control System Faults1. The brake pedal has no free travel, meaning the driver is effectively pressing the brake continuously. In this case, the free travel of the brake pedal needs to be adjusted. Generally, the free travel should be between 10-15mm, and a slight gap should be felt when pressing the brake pedal by hand.
2. Internal faults of the brake master cylinder, such as a stuck piston, broken spring, aging or damaged rubber seals, or excessive wear, which can cause air leakage or prevent the piston from returning promptly.In general, brake master cylinders are not repairable. If the above faults occur, the only solution is to replace the entire unit. For some vehicle models, if the internal rubber seals of the brake master cylinder are excessively worn, the sealing surface can be ground on a flat surface and then the brake clearance can be readjusted.However, this requires a high level of technical skill, and the reliability and durability after repair are generally poor. It is generally not recommended, but it can be used for temporary emergency situations.
3. Internal failure of the relay valve. Nowadays, most pneumatic brake systems use relay valves as intermediate components in order to shorten braking time and release brakes quickly.If the piston inside the relay valve gets stuck, the spring breaks, or the rubber sealing components wear out excessively or age, it can also cause brake air pressure to continuously push the brake wheel cylinder, resulting in brake drag.Relay valves generally cannot be repaired. If a failure occurs, the assembly must be replaced. In short-term emergencies, it is possible to grind the sealing components like repairing a brake master cylinder, but the effect is not very good.
4. Internal failure of the brake wheel cylinder, such as broken internal springs or a stuck piston, generally requires replacement of the entire assembly and repair is not recommended. Especially for modern trucks, which usually use heavy-duty spring brake wheel cylinders, disassembling the brake wheel cylinder without specialized tools could even lead to serious safety accidents.There is also a relatively special situation: after dismantling and reinstalling the brake wheel cylinder, if the service brake and parking brake air hoses are connected incorrectly, it can cause the parking brake to release during driving while the air pressure flows to the service brakes, thereby causing brake drag. This situation is not uncommon.
▎2. Wheel Brake MalfunctionWheel brake malfunctions are generally quite simple. By disassembling the brake, the faulty part can usually be identified. The vast majority of malfunctions are caused by rust on the brake shoe shaft. Simply removing the brake shoe, cleaning off the rust, and re-lubricating it can resolve the issue.In addition, a broken or weak brake shoe return spring, rusted or excessively worn brake cam, or a faulty brake adjusting arm can also cause brake dragging. These parts require regular maintenance and lubrication. The so-called secondary maintenance for trucks mainly involves the lubrication and maintenance of the wheel brakes.
The above is an analysis of the common causes of brake lag in air-pressure brake trucks. In practice, faults in the air pressure control system are relatively rare, but if they occur, they are also the most difficult to diagnose.
The wheel brakes mainly require proper maintenance and lubrication, especially after driving through water in the summer. Timely maintenance of the wheel brakes is necessary, otherwise the brake shoe shafts can easily become corroded.
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