Bench bleeding a master cylinder

After removal, repair, or replacement of the main brake cylinder performs bleeding of a brake master cylinder on the workbench. Bench bleeding makes removing air from the hydraulic system easier when the master cylinder is installed in place. Otherwise, the air in the cylinder reaches the hydraulic system directly, preventing pressure from being created and making it difficult to bleeding.

First, the old brake fluid is removed from the tank. Then all brake lines and the nut that connects the master brake cylinder and a vacuum brake booster are removed. The master cylinder pushrod length is measured.

The master brake cylinder is removed and mounted on a vise so that the paws cover the cylinder lugs. Install the bleeder lines into the master cylinder outlet ports with the ends of the lines deep within the master cylinder reservoir. Fill the tank halfway with clean brake fluid.

Bench bleeding a master cylinder

Using a suitable tool, gently push the piston into the brake cylinder. The pressure created inside the cylinder should push air and old brake fluid through the hoses into the tank. Release the piston to return to the starting position. Repeat the procedure until all air bubbles are removed from the master cylinder.

After bleeding the master brake cylinder on the workbench, we mount the master brake cylinder on the vacuum brake booster and fasten it with screws. Remove the hoses from the cylinder and connect the brake lines. We add brake fluid to the tank. The master cylinder is now ready for operation on the brake system.

To bench bleed a master cylinder, follow these steps:

Removal from the vehicle

Remove the old brake fluid from the reservoir. Remove all brake lines from the cylinder. Unscrew the nut that connects the cylinder and the vacuum brake booster. Remove the cylinder from the vehicle. Measure the master cylinder pushrod length.

Preparation for bleeding

Remove the removed cylinder to the vise. Connect the air extraction hoses to the hydraulic connections of the brake cylinder. Insert the other end of the hose into the tank. Fill the tank halfway with clean/new brake fluid.

Bleeding

Gently depress and release the brake cylinder piston and observe the brake fluid passing through the bleeding hoses from the cylinder into the reservoir. Repeat the procedure until the air bubbles are removed from the brake fluid.

Installation on the vehicle

Connect the master brake cylinder to the vacuum brake booster and secure it with nuts. Remove all air extraction hoses and connect the brake lines to the brake cylinder. Top up the brake fluid in the reservoir to the appropriate level.