Brake Master Cylinder

The braking system in most vehicles requires hydraulic pressure to operate. The brake master cylinder is a component that converts the mechanical force applied to the brake pedal into this pressure. It is essentially a short tube containing two small pistons, with a reservoir filled with brake fluid attached to the top. The cylinder and reservoir are designed to create two separate chambers so that a leak in one brake line will not affect the other brakes. A brake master cylinder can last for many years but will eventually wear out and need to be replaced.

The first stage of the braking system used in most vehicles is the brake pedal, brake booster and brake master cylinder. The booster and cylinder are usually located just behind the pedal. The first piston inside the cylinder is connected to the booster and pedal by steel rod linkages. The second piston sits between two springs and moves with the first piston. The cylinder is connected to the brakes by two brake lines that are attached to the side of the cylinder. The body of the cylinder is normally mounted to the vehicle body with two small bolts. Most cylinders also have a sensor and two wires connected to the reservoir to measure the brake fluid level.

When the brake pedal is depressed, the pistons in the brake master cylinder move forward and push fluid through the lines to operate the brakes. When the brake pedal is released, the pistons move in the opposite direction and fluid flows back into the cylinder and reservoir. The high pressures that are created inside the cylinder and brake lines can cause brake fluid to leak out of small cracks and loose joints. By having springs between the pistons, one brake line can still work if there is a loss of pressure in the other line. The only noticeable difference to the driver is the reduction in braking power and the need to press the brake pedal further to stop the vehicle.

Replacing the brake master cylinder is a moderately simple task which can be done by most people with a few common tools. An adjustable wrench or spanner set is required to remove the bolts connecting the cylinder to the vehicle body. They are also needed to remove the nuts connecting the linkages and brake lines to the cylinder. The brake fluid in the reservoir should be removed before disconnecting the brake lines. This can be done with a siphon, syringe or even a turkey baster. Brake fluid is corrosive and should not be allowed to touch the paintwork, but it is also water soluble and can be removed quickly if this happens. The plug for the brake fluid level sensor also needs to be detached before the cylinder can be removed.

The replacement brake master cylinder first needs to be bleed of air before it can be installed. This is done using a bleeder kit that is usually included with a new cylinder. Plastic tubes are placed inside the cylinder and also connected to the brake line ports. New brake fluid is added to the cylinder and the piston operated until all the air bubbles have been removed. Once this has been done, the ports are plugged and the bleeder kit removed. The cylinder is then mounted back in the vehicle and the brake lines and linkages reconnected. After a new cylinder has been installed, the brakes should be bled as normal.

Always refer to the vehicle service manual before attempting any work on the braking system. There can be many difference between the various makes and models, and what works for one vehicle may not work for another. Before using the vehicle on the roads, check that the braking system works and that there are no fresh leaks coming from the cylinder or brakes. This is especially important after replacing the brake master cylinder because both sets of brakes depend on it working normally.

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