How to Bleed Air Out of a Master Cylinder

Any time you are working on your vehicle's brake system, you must remove any air trapped within the system so the brakes work properly. This is especially true if you have removed and are re-installing the brake master cylinder. Bleeding the master cylinder is done in two parts. You must "bench bleed" it before installation and then bleed it again once it's in place within the vehicle.

Instructions

1

Place the master cylinder within a vise and clamp the jaws onto the master cylinder's mounting flange to hold it. Connect a pair of bleeder tubes which go from the outlet ports on the side of the cylinder to the reservoir on top. Fill the cylinder with the correct type of brake fluid for your vehicle.

2

Press the pistons on the cylinder slowly to expel the air into the reservoir. Use a Phillips screwdriver or similar tool and look for bubbles in the reservoir as you press. Release the pistons and continue to press and release until you see no more air bubbles in the reservoir.

3

Remove the bleeder tube from one outlet port and quickly plug that port with a rubber plug to prevent the fluid from escaping. Repeat this with the other outlet port and then remove the bleeder tubes from the reservoir.

4

Install the master cylinder in the vehicle; place it over the studs on the brake booster with a new gasket and tighten the nuts by hand. Remove each plug from the outlet port one at a time and quickly replace it with the correct brake line. Tighten the cylinder's mounting nuts with a wrench and make sure the cylinder is filed with the correct amount of fluid.

5

Bleed the air out of the brake lines at the cylinder. Have an assistant press on the brake pedal while you loosen the fitting connecting one brake line to the cylinder. Tighten the fitting once there are no air bubbles escaping, then repeat for the other brake line.