GM 18047594
Master Cylinder

When fixing the Brake Master Cylinder start by clamping its mounting flange in a workshop vise so you can reach the rear of the main Piston. Set up suitable brake fittings to the master cylinder ports for hose attachment following flare seat specifications. Install transparent hoses to these fittings and lead them to the master cylinder reservoir. Put delco supreme 11(R) brake fluid gm p/n 12377967 or its dot-3 brand equivalent into the reservoir through one end of the transparent hose lines until the fluid reaches half the container while keeping both hose tips fully under liquid. Use a rounded-end tool to control the primary Piston up and down multiple times; look at the fluid flowing from the ports. At the same time the primary Piston becomes harder to press and moves less with each bleed cycle. Repeat this step until clear fluid comes out smoothly from both ports without air present. When complete remove the transparent hoses from the reservoir then replace the master cylinder parts before disconnecting the hoses from their ports and putting down the master cylinder with a shop cloth to prepare for removal.

Draining air from the Brake Master Cylinder through bench bleeding saves time during the brake hydraulic system bleeding process. Set the master cylinder in a vise with slight tilt from the front and connect the ports to the vacuum source. Progressively push the primary Piston 25 mm as air escapes by using a round tool on the cylinder reservoir filled with new brake fluid. Refit the master cylinder into the vise with an opposite tilt and restore the same Piston stroke twice. Fill the reservoir with fluid and put the master cylinder into the vise with an even base. Take the outlet port plugs loose one by one and push the Piston toward the bore to remove trapped air. Seal the plug again before letting the Piston return naturally because air can enter the cylinder if left open. After filling the reservoir with fresh fluid put the master cylinder in place and continue the simple brake bleeding procedures.

Plug the cylinder inlet ports and the rear outlet port, then apply low pressure, non-lubricated, filtered air into the front outlet port to remove the secondary piston along with the primary and secondary seals and the return spring.

Bleed the hydraulic brake system.

Remove the master cylinder nuts and take out the master cylinder.

Lubricate the grommets, seals, and cylinder bore with clean brake fluid, using new seals.

Bench bleed the master cylinder.

Clean the area around the reservoir cap.