Car brake master cylinder - with oil reservoir function
The brake master cylinder (also known as the brake main cylinder) is the core component of the braking system. Its main function is to convert the mechanical force applied by the driver on the brake pedal into hydraulic pressure, and then transfer this pressure through the brake oil lines to the brake booster of each wheel, thereby pushing the brake pads to clamp the brake discs (or brake shoes to press the brake drum), achieving vehicle deceleration or stopping.
The brake master cylinder is usually integrated with a component called the "brake oil reservoir" (or fluid reservoir). The reservoir is not an independent functional component but an important part of the master cylinder system. Its functions are as follows:
- Storing brake fluid: providing sufficient brake fluid for the entire hydraulic braking system to ensure there is enough fluid to transmit pressure during braking.
- Maintaining fluid level balance: as the brake pads wear, the piston in the brake booster moves outward, causing a relative decrease in the amount of brake fluid in the system. The oil in the reservoir will automatically replenish the pipeline, so the liquid level of the reservoir will slowly drop as a normal phenomenon.
- Sealing and anti-pollution: the reservoir cover has a sealing function, which can effectively prevent moisture and impurities from entering the brake fluid system. If moisture seeps in, it will lower the boiling point of the brake fluid, causing air blockage at high temperatures, and in severe cases, leading to soft brakes or even failure.
- Ventilation balance: the top of the reservoir has an air vent, which can maintain the pressure balance inside and outside the system and avoid negative pressure causing the brake fluid to not flow normally.
- The working process of the brake master cylinder and the reservoir:
When the driver steps on the brake pedal, the push rod pushes the piston inside the master cylinder forward.
The piston compresses the brake fluid, creating high pressure in the sealed pipeline.
The high-pressure brake fluid is rapidly transmitted through the oil lines to the four-wheel booster.
The piston of the booster pushes the brake pads to contact the brake disc, generating friction to achieve braking.
After releasing the pedal, the piston returns to its original position under the action of the return spring, and the brake fluid flows back through the compensation holes on the master cylinder, releasing the system pressure.
If the brake master cylinder or the reservoir fails (such as oil leakage, aging of the rubber seals, or failure of the cover seal), it will cause the brakes to become soft, the pedal to be unable to be fully depressed, the braking distance to increase, or even complete failure, posing serious safety hazards. Therefore, it is crucial to regularly check the brake fluid level, color (should be clear and transparent), and replacement cycle (recommended every 2 years or 20,000 kilometers).
The brake master cylinder (with reservoir) is the core component of the braking system, and its failure will directly affect driving safety. According to current public information, brake master cylinder failures often accompany reservoir-related abnormalities, mainly manifested as the following symptoms:
Main fault manifestations
- The brake pedal becomes hard or cannot be depressed: The sealing ring inside the master cylinder is worn or stuck, causing the brake fluid pressure to fail to be established normally, and the pedal feedback becomes hard, resulting in brake failure.
- The brakes feel soft and the travel lengthens: The sealing parts of the master cylinder are aging or there is internal leakage, causing insufficient hydraulic system pressure, and the pedal feels "soft" when depressed, resulting in an extended braking distance.
- The brakes do not return to position or return slowly: The return spring of the master cylinder fails or is internally blocked, causing the brake pads not to fully separate from the brake disc, possibly causing drag braking or overheating.
- The brakes veer, shake, or make abnormal noises: The output pressure of the master cylinder is uneven, causing an imbalance in the braking force of the left and right wheels, or internal parts wear and produce abnormal noises.
- Abnormal drop in reservoir fluid level or oil leakage: The rubber seals of the master cylinder are damaged, the reservoir seal is poor, or the connecting pipeline is loose, which will cause brake fluid leakage, a drop in fluid level, and even trigger the warning light on the dashboard.
- The brakes suddenly fail: In severe cases, the master cylinder completely fails, and depressing the pedal has no braking force, which is an extremely dangerous fault.
Common causes
- Aging sealing parts: The rubber seals and sealing rings inside the master cylinder are exposed to heat and pressure for a long time, will harden and crack, resulting in internal leakage or external leakage. Excessive moisture content in brake fluid: The top of the brake fluid reservoir has a ventilation hole. Brake fluid is prone to absorbing water. If the moisture content exceeds 3%, it will reduce the boiling point and easily vaporize at high temperatures, forming "air resistance" and causing brake failure.
Loose or damaged connections: The connection pipes and joints between the oil reservoir and the master cylinder are loose or broken, causing oil leakage.
Fault of the vacuum booster pump: The master cylinder and the vacuum booster pump are closely linked. If the booster pump fails, it will increase the pedal force, but it will not directly cause oil leakage from the reservoir. A distinction in diagnosis is required.
Suggested treatment methods
Stop the vehicle immediately for inspection: If any of the above abnormalities occur, stop the vehicle safely and do not continue driving.
Check the brake fluid level: Open the engine hood and check if the reservoir level is between "MIN" and "MAX". If the level is too low, check for any leakage points.
Professional diagnosis and replacement: The brake master cylinder is a key safety component. Do not attempt to disassemble it yourself. Immediately go to a 4S store or a professional repair facility, use a diagnostic tool to read the fault code, and replace the original or quality-certified master cylinder assembly.
Synchronized maintenance: When replacing the master cylinder, it is recommended to also replace the brake fluid (fully drain and clean the entire vehicle), and check the wear of the brake pads and brake discs.
Important reminder: The braking system is related to life safety. Any abnormality must not be ignored. Timely professional maintenance is the only safe option.
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