Instructions to Change a Master Cylinder

The master cylinder is the heart of your vehicle's brake system, pumping hydraulic fluid to the brake caliper or wheel cylinder at each individual wheel. Over time, the seals inside the master cylinder can begin to leak, reducing the effectiveness of you vehicle's brakes. Fortunately, master cylinder replacement is a relatively straightforward process that's well within the abilities of the home mechanic.

Location

On most cars and light-duty trucks, the master cylinder will be bolted to the firewall on the right side (when standing in front of the vehicle and looking towards the windshield). You can quickly identify it by the steel brake lines that exit the cylinder on the bottom.

Removal

Begin by removing the cap and removing the brake fluid from the master cylinder reservoir. Most modern master cylinders incorporate a dual reservoir design, meaning there is a separate hydraulic circuit for the front and rear brakes. Make sure to empty both reservoirs. Remember that brake fluid can strip the paint of finished surfaces, so be careful not to let any fluid contact the painted surfaces on your vehicle. Next, remove the steel brake lines from the bottom of the master cylinder. Be careful not to damage the lines while removing them. There are usually two to three bolts that secure the master cylinder to the power brake booster. Remove these bolts at this time. Depending on the design of your brake system, you may or may not have to disconnect the brake pedal pushrod from the brake pedal. Once the master cylinder is free, remove it from the vehicle.

Installation

Begin by lifting the new master cylinder into position and reinstalling the two or three bolts that connect it to the power booster. Reconnect the brake pedal pushrod if it was disconnected during removal. Reinstall the steel brake lines that connect to the bottom of the master cylinder, but be careful not to put too much stress on the lines, as they can break easily.

Brake Bleeding

Brake bleeding is the process of removing air from the hydraulic circuit. Brake fluid for all intents and purposes is not compressible, but air is. If their is air present in the hydraulic circuit, the input force applied when you press the brake pedal will be used to compress the air in the lines and not in pushing the the brake pads against the brake discs or drums. Begin by filling the master cylinder. Always use clean brake fluid and never shake the bottle before topping off the master cylinder. This will only introduce air into the fluid. Be sure not to let the master cylinder run dry at any point in the brake-bleeding process. Doing so will only introduce more air into the system. If there is a bleeder valve on the master cylinder bleed that valve first. Push a length of clear plastic hose onto the bleeder valve. Place the other end of the hose in a container filled with brake fluid. Make sure this end of the hose is covered by brake fluid at all times. Have an assistant working inside the vehicle pump the brake pedal up and down a few times and then hold the pedal down. As you open the bleeder valve, your assistant will notice the pedal move towards the floor. Close the valve before the pedal bottoms out. As air is purged from the system you will be able to see small air bubbles in the length of clear plastic hose. Repeat this process until there are no more bubbles in the tube. Once you've bled the master cylinder, move to the wheel closest to it. On most vehicles this will be the driver's side front wheel. A bleeder screw will be located on the back or top of the brake caliper close to where the rubber brake line attaches. Bleed the caliper using the same process you use on the master cylinder. Then move to the next closest wheel. On most vehicles this will be the passengers side front wheel. Most vehicles will be equipped with drum brakes on the rear axle. The bleeder screw will be located on the inboard side of the brake drum backing plate. Bleed the driver's side rear wheel cylinder, and lastly the passenger side wheel cylinder.