When installing a new or refurbished master cylinder, it is critical that air is removed from the unit. This can easily be done by bench bleeding the master cylinder prior to installation.
Step 1: Place your master cylinder in a vise by the mounting ears (not the body)
Step 2: Attach a short piece of brake line to each fitting.
Step 3: Arrange the lines so they are submerged in the reservoir on either side of the wall.
Step 4: Repeat steps 2 and 3 on the other brake line port hole.
Step 5: Fill the reservoir with clean brake fluid recommended by the manufacturer.
Step 6: Using full strokes, push the piston in, then release. Do this until the air bubbles no longer appear in the brake fluid.
Note: The master cylinder will not bleed properly unless the hoses are fully submerged into the brake fluid. The hoses must remain submerged until the bleeding process is complete.
Now mount master cylinder and avoid brake fluid leaking out of front and rear ports during installation.