​​​​​​​Master Cylinder Bench Bleeding

When replacing a new master cylinder, it is beneficial to bleed any air out of the cylinder before installing it in the vehicle. This will save a lot of time and heartache when bleeding the brake system. There are a few different methods for bench bleeding master cylinders. The traditional method is covered here for the purpose of simplicity. This method provides a way to visually verify that no air remains in the master cylinder. New master cylinders will usually come with a kit like the one pictured here. It is a good idea to save this kit for later use in case you purchase a master cylinder that doesn’t come with one, or if you need to bleed an existing master cylinder in the future. Block off plugs can come in handy when diagnosing a hydraulic fault. If the pedal is hard with the block off plugs installed on the master cylinder, then the problem is somewhere else in the hydraulic system. If your master cylinder does not include the kit, there are several bleeder kits available, or you can you can use some clear plastic hose which fits tightly into the threaded outlets.

Use a non-cloth fender cover to protect the paint. Place the old master cylinder in a catch pan to avoid spilling fluid. Be careful not to spill fluid on any painted surfaces, as brake fluid will rapidly deteriorate the paint. If any brake fluid is spilled, immediately flush the affected area with water and wipe it clean with a dry towel.

Remove the reservoir from the old master cylinder and clean it if necessary. Do not use any type of petroleum solvents, as they will cause the rubber components to swell. Use compressed air to clear out any moisture and debris. When swapping the reservoir to the new master cylinder, lubricate the reservoir seals with clean brake fluid to ease installation. Secure the new master cylinder in a vise, and make sure that it is level. If the cylinder is not level, air can become trapped inside. Always clamp the master cylinder on the flange area as shown, and never clamp the master cylinder anywhere on the body area.

Begin pumping the master cylinder slowly with a blunt tool, such as a socket extension or a dowel. You will see bubbles of air escape through the lines. Top off the fluid if necessary, so that the hoses remain submerged in fluid. If the hoses do not remain submerged in the fluid, air will be drawn back into the cylinder.

Continue pumping until there is no more air in the lines. The lines should look like this.

Install the reservoir cap, then install the block off caps on the outlet ports. Don’t forget to install the vacuum booster gasket or o-ring.

Install the master cylinder onto the booster, but don’t tighten the mounting bolts completely. Allow a little wiggle room to make threading the brake line fittings a bit easier. Remove one plug at a time, and attach the lines. Tighten the master cylinder mounting bolts and the brake lines. Bleed the brakes according to the manufacturer’s procedure, making sure to maintain the fluid level in the reservoir. It is a good idea to flush all of the old fluid from the hydraulic system at this point. It should take minimal effort to bleed any air out of the system now that the master cylinder is primed.


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