GM 19209201
Master Cylinder

Make sure you are safe before working on the brake master cylinder by reading the Brake Fluid Irritant Caution and being aware of how brake fluid may affect paint and electrical parts. Make sure to use only Set the brake master cylinder's mounting flange into a bench vise so you can work on the back of the main piston. First, remove the master cylinder reservoir cap and diaphragm. Then, put suitable fittings onto the master cylinder ports for hose connection. Put transparent hoses on the fittings and bring them into the reservoir, making sure to block the clutch hose port if working on a manual transmission. Put enough fresh brake fluid in the reservoir until it reaches halfway, making certain the hose ends are covered. With a gentle, round-tipped implement, make the primary piston move in and out about 25 mm a few times and look for the fluid running from the ports. you will find the piston harder to push and requires more force as air escapes, while the movement gets shorter. Keep going with this until the fluid runs smoothly out of the ports without any air bubbles. Then, take off the transparent hoses, fit the reservoir cap and diaphragm back on, and take out the fittings after wrapping the master cylinder in a clean cloth to stop any fluid from leaking. When you're done, take the master cylinder out of the vise.

Secure the mounting flange in a bench vise, ensuring the rear of the primary piston is accessible.