The brake master cylinder, together with the brake booster, provides the necessary brake pressure in the vehicle’s brake lines. Every time the brake is stepped on, the component is subjected to load. If the brake master cylinder is defective, the required braking power is no longer available. Find out everything you need to know about the function and replacement of the master brake cylinder in our guide.

What is the function of the brake master cylinder?

A master brake cylinder is one of the main components of the hydraulic brake system. When the driver presses the brake pedal, the brake cylinder converts the pedal force into brake pressure. To do this, a pressure piston moves inside the cylinder. The pressure generated is transmitted via brake lines to the brake pistons in the brakes. When the driver takes his foot off the pedal, the piston returns to its original position through brake pressure and spring force.

Greater safety with the tandem brake master cylinder

Every modern car has a so-called tandem master brake cylinder for two brake circuits. In this type of master brake cylinder, two pistons are arranged in one cylinder. Pressure is generated independently for each brake circuit. If one of the two brake circuits should ever be damaged, each of the circuits continues to operate independently.

Master brake cylinder defective – these are the causes

As part of the braking system, the master brake cylinder is loaded during every braking operation. When the brake pedal is stepped on, pressure builds up in the pressure chambers. Releasing the brake is accompanied by a reduction in pressure. With high mileage, this can lead to wear on mechanical parts such as the piston and the actuating rod. This is a common cause of brake master cylinder failure. Leaks in the brake master cylinder can also lead to failure of the component. The structure inside the cylinder includes sleeves and seals. These seal the pressure pistons of the brake circuits against each other. The seals also prevent brake fluid from leaking through the actuating mechanism. Over time, the seals can become porous or worn, causing the master cylinder to become defective.

Damage due to weather or lack of maintenance

In a car, the brake master cylinder is located directly in the engine compartment along with the brake booster and the brake fluid reservoir. Here, the components are exposed to weather conditions such as moisture, which can lead to rust and damage. Lack of maintenance of the brake system or changing the brake fluid too infrequently can also be the cause of a defect in the master brake cylinder. Overaged brake fluid absorbs water, which is sometimes the cause of corrosion throughout the system.

Common causes of brake master cylinder defects:

• Wear with high mileage
• Defect of the seals inside
• Damage due to weather conditions
• Rust on the brake master cylinder
• Maintenance of the brake system has not been carried out
• Brake fluid has not been changed

Defect at the master brake cylinder – these symptoms appear

A defective master brake cylinder often draws attention to itself through typical symptoms. This changed braking behavior is usually noticed quite quickly by the driver. The defect in the master brake cylinder is often accompanied by reduced braking force. The car has a longer braking distance and does not stop as usual when the brake pedal is pressed. If the master brake cylinder is defective, the pedal sometimes does not return to its original position. A long pedal travel is also one of the typical symptoms, with the braking effect occurring late. A defective and leaking cylinder sometimes leads to a loss of fluid in the brake system. This visibly leaks or is missing from the expansion tank.

Typical symptoms at a glance:

• Braking effect is noticeably reduced
• Pedal travel is extended
• Pressure point is undefined
• Leakage of brake fluid
• Brake pedal does not return to initial position
• Brake fluid in the reservoir sinks

No pressure when stepping on the brake pedal due to defective master brake cylinder

When braking a car, the pedal often feels different when the brake master cylinder is defective. Sometimes the driver misses a clearly defined pressure point, or the pressure in the brakes is available later. The brake circuits may also respond differently if there is a defect in the master cylinder. If there are symptoms of a defect, the only solution is usually to replace the master brake cylinder.

Replacing the master brake cylinder and buying suitable spare parts

If the brake master cylinder is defective, it is usually possible to replace the entire part. A repair involving disassembly is usually only worthwhile for exotic vehicles for which no suitable spare part is available. When buying a master brake cylinder, make sure that the purchased part fits the car exactly. Brake master cylinders for cars with ABS control unit or for drum brakes may have a different design. As part of the brake system, the brake master cylinder is responsible for safety while driving. Repairs to the brake system should only be carried out by expert hands or together with a professional as an advisor. Any mistake in changing the brake components can quickly lead to dangerous situations. A specialist workshop from the region is the better choice if there is no knowledge about the structure of the brake.

Changing the master brake cylinder

The replacement of the master brake cylinder by a professional is always carried out in compliance with all safety regulations. The procedure may differ depending on the vehicle model. Escaping fluid must be collected and disposed of properly.

1. Prepare for repair: Before the master brake cylinder can be replaced, the repair must be prepared. This often involves dismantling parts in the engine compartment. The brake fluid is siphoned off or drained via the expansion tank.
2. Remove the master brake cylinder: The next step is to remove the defective master brake cylinder from the car. To remove the component, all connections and connecting hoses must be loosened. The same applies to the component’s screw connections.
3. Compare component: Once the master brake cylinder has been dismantled, the component can be removed from the engine compartment. Any fluid adhering to it is collected with a cloth. It is then a good idea to compare the defective component with the spare part.
4. Install the spare part: If the fit is correct, the intact component is installed in the engine compartment. It must be fastened in accordance with the manufacturer’s instructions. The lines and connections are then reassembled on the replacement part.
5. Bleeding the brakes: After the procedure, the missing brake fluid in the system is topped up. This should involve a complete change of fluid. To ensure reliable functioning of the brake system, the system must then be bled.

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