When installing a master cylinder, it is critical to remove all of the air from the system before proceeding. This can result in poor braking performance if you don’t do it. If you have a mushy or non-responsive brake pedal, you may have to start over from the beginning. Master cylinders with step bores are more difficult to bench bleed than master cylinders with standard bores. They make use of an internal valve known as a rapid take-up valve, which retains air and makes bleeding more challenging. As a result of the bigger primary bore, step bore master cylinders require longer refill times while bench bleeding.
When bleeding a step bore master cylinder, it is preferable to use a brake syringe, or master cylinder bench bleeder tool, rather than a brake syringe. When utilizing this instrument, the initial step is to clamp the master cylinder to the mounting flange of a vice. This will allow you to bench bleed the cylinder without damaging it. Make sure the mount is level, and avoid clamping the mount under the casting body, as this could harm the bore of the cast iron casting. To catch any fluid that may be discharged during the bleeding process, install a drain pan underneath it. Removing the protective caps from the primary brake line port and refilling it with fresh, clean brake fluid until it is at least half full is then necessary. Draw some fluid into the syringe and then expel the air out of the syringe to complete the procedure. Inject brake fluid into the reservoir, starting with the primary in-line, until clear, non-aerated fluid flows through the primary brake line port. Then, refill the syringe with fluid, purge the air out of it once again, and inject the fluid into the primary brake line port until there are no more air bubbles visible in the tank. Complete the installation by following the procedures outlined in your OE-approved service manual for the secondary in-line and secondary brake line connections.
You can still perform a traditional style bench bleeding operation if you don’t have a suitable syringe on hand; however, this will need stroking at the piston and will be far more time-consuming. The procedure for bleeding a step bore master cylinder differs slightly from that of bleeding a standard master cylinder. Pay great attention to the process as a result.
Install the master cylinder in a vice by the mounting flange, this time with the left side facing up. Once this is completed, insert the supply plugs into the primary and secondary brake line ports of the master cylinder, and fill the reservoir halfway with new, clean brake fluid. Depress the piston with a blunt object in a short stroke of an inch or less and hold it for 20 seconds before releasing it. Keep in mind that the stroke should not be longer than one inch, or else you risk damaging the seal. The key to getting all of the trapped air out of the rapid take-up valve is to keep the piston depressed throughout the process. The bigger primary bore must be allowed to refill the fluid between each stroke; thus, it is critical that you wait at least 30 seconds between each. Repeat as necessary until there are no visible air bubbles and the piston travel is less than 1/8th of the initial stroke. Finally, complete the installation according to the instructions in your OE-approved service manual.