What is a brake master cylinder and where is it located?

The brake master cylinder is an important part of the braking system. It applies hydraulic pressure to the brake pistons to slow down the vehicle’s wheels. When the driver presses on the brake pedal, it pushes forward a pushrod, which in turn pushes a set of pistons inside the master cylinder. This pushes out brake fluid, which creates hydraulic pressure. Fluid hoses carry that pressure to the brake pistons at each wheel. In disc brakes, the caliper pistons squeeze the pads into the rotors to slow the wheels. In drum brakes, the wheel cylinder pistons push the brake shoes into the drums to slow the wheels. 

The brake master cylinder actually contains two cylinders with two pistons, known as a tandem master cylinder. This is how it sends fluid to the front and the rear brakes. The pressure generated by the first cylinder pushes the piston in the second. Fluid will flow through both lines, activating the front and rear brakes. Internal valves determine how much pressure is sent to each brake. 

The brake master cylinder can be found inside the engine compartment, attached to the firewall, opposite the brake pedal. In a manual transmission vehicle, it is next to the clutch master cylinder, further to the passenger’s side of the vehicle. The fluid is contained in a plastic reservoir, which is often mounted on top of the master cylinder. 

How do I know if my brake master cylinder needs to be replaced?

Over time, the seals inside the master cylinder can break down from use and cause fluid to leak out of the system. Signs of fluid leaking along the firewall or inside the vehicle by the brake pedal will indicate that the brake master cylinder needs to be replaced. Without proper fluid levels in the system, the brake pedal can feel soft and not allow for proper movement of the brake. A soft pedal is an early sign that something can be wrong with the brake master cylinder. If the pedal feels “spongy” when you press it, this can indicate that there is air in the hydraulic system. If the pedal sinks to the floor and stays there when you take your foot off, then there is definitely a leak in the brake system.

Because of the potential danger to yourself or other drivers, any problems with your brakes should be diagnosed and fixed as quickly as possible.

Can I replace a brake master cylinder myself?

Yes, it is possible to change your own brake master cylinder with basic automotive knowledge and tools. Be sure to have a catch pan ready for any fluid that may drain when you disconnect the master cylinder from the fluid reservoir. Once that is done, you’ll have to disconnect the master cylinder from the brake pedal and unbolt it from the firewall. You’ll also have to disconnect the wiring harness for the low fluid sensor if one is equipped. Some replacement brake master cylinders come with new brake fluid reservoirs, while others do not. If yours does not come with one, you will have to separate the fluid reservoir from the old master cylinder and reuse it. You may also have to reuse a valve that sits between the pushrod and the master cylinder.

When all that is done you can mount and reconnect the new master cylinder, refill the hydraulic fluid, and bleed the system. After doing so, you’ll need to bleed the brake hydraulics; this may require the help of an assistant if you do not have a vacuum bleeder.

Need a brake master cylinder replacement?

Your brake master cylinder plays a major role in your vehicle’s braking system, a system of which its importance cannot be overstated. Any time a part of your braking system begins to fail, it is a major safety concern and it should be replaced immediately. At 1A Auto, we carry a large selection of replacement brake master cylinders for many makes and models, and at great prices. Brake master cylinders from 1A Auto are all new, not rebuilt. They are made to original factory specifications for a trouble free direct replacement.

We also make shopping for a replacement brake master cylinder for your car, truck, SUV or van easy — we're here to help you select the right part for your vehicle! Call our customer service toll free at 888-844-3393 if you have any questions about our parts, warranty, compatibility or to purchase, or you can buy online.