Why does the indicator light still stay on after releasing the air brake?
4 Answers
When the indicator light remains on after releasing the air brake, it may be due to poor circuit contact or the brake not being fully released. The air brake is a type of braking system. In automobiles, a series of specialized devices that apply a certain force (primarily on the road surface) to specific parts of the vehicle (mainly the wheels) to enforce a certain degree of braking are collectively referred to as the braking system. The car's brake pedal is located below the steering wheel. Pressing the brake pedal activates the brake lever linkage, which transmits pressure to the brake shoes on the brake drum, clamping the brake disc and causing the vehicle to decelerate or stop. The manual brake in a car is located beside the gear shift and is connected to the brake lever.
As a veteran heavy truck driver with over a decade of experience, I've encountered the issue of the parking brake warning light staying on twice. The first time it happened was truly frightening - I was terrified of losing braking power on the road. The most common cause is insufficient system air pressure, which could be due to reduced compressor efficiency, minor air leaks in the tubing, or an improperly closed air tank drain valve. If the light comes on right after starting the engine, it's normal since the air pressure hasn't built up yet - just wait patiently. But if it stays on after driving for a while, you need to investigate immediately. Once I ignored it while rushing for delivery and nearly caused an accident. My experience tells me this warning light is no joke - if it stays illuminated, never force the vehicle to move. Pull over immediately to thoroughly check the air system for any hissing leaks and verify if the pressure gauge needle reaches above 6 kg/cm². If you find the pressure won't build up, absolutely don't operate the vehicle - calling a tow truck is far safer than taking the risk.
Over the years of repairing truck brake systems, I've handled no less than a hundred cases of abnormal activation of the spring brake warning light. Aside from the common issue of insufficient air pressure, issues like a stuck return spring in the hand brake control valve or a clogged exhaust valve can also cause signal transmission errors. Another frequent culprit is a short circuit in the dashboard wiring, especially in vehicles with frequently tilted cabs where worn-out wires send erratic signals. The most easily overlooked problem is a faulty air pressure switch, which may falsely indicate low pressure even when it's sufficient. In such cases, I usually start by spraying soapy water on the air lines to check for leaks, then use a multimeter to test the switch's on/off state. However, I advise vehicle owners not to attempt disassembly without proper tools, as improper handling might accidentally trigger emergency braking. The safest approach is to visit a certified repair shop to read the fault codes with a diagnostic scanner, especially since it concerns brake safety.
From a technical perspective, the continuous illumination of the warning light after releasing the spring brake is a typical signal feedback anomaly. The core principle is that the system does not receive a confirmation signal indicating the complete release of the brake chamber. There are generally three fault paths: First, the air pressure path issue—when the reservoir pressure falls below the safety threshold (commonly 4.5 bar), the pressure sensor will continuously trigger an alarm. Second, mechanical path failure—if the pushrod of the spring brake cylinder does not travel sufficiently, it will trigger an abnormal mechanical limit switch. Lastly, electrical circuit faults, including a short circuit in the dashboard light control circuit, grounding of the signal feedback line, or misjudgment by the control module. It is recommended to troubleshoot in the following sequence: observe the readings on the reservoir's dual-needle pressure gauge → check the reset status of the brake chamber pushrod → test the continuity of the limit switch → and finally measure the resistance of the wiring harness.
