Brake Master Cylinders
About Brake Master Cylinders
The master cylinder is the hydraulic pump on the lever or pedal. When you squeeze or press, its piston pushes fluid down the line to the caliper. Riders replace one when it leaks, when the lever feels soft after the lines and caliper have been checked, or when they want a different style or lever feel.
The key spec is bore size, shown in listings in fractions such as 9/16 inch, 5/8 inch and 11/16 inch. A smaller bore produces more line pressure for the same lever effort but needs more lever travel, and a larger bore does the opposite, so the bore should suit the caliper. Next is position: front, rear, or a hydraulic clutch master cylinder, which is listed separately. Finish and styling vary, with black, chrome and polished versions, and some listings are complete assemblies while others are kits. Cylinders are also model-specific, so check the fitment in the title.
Brands in this category include Braking, Drag Specialties, Performance Machine (PM), Works Connection and Moose Offroad. A worn cylinder can sometimes be rebuilt with parts from Rebuild Kits & Parts, and caps and covers are in Reservoirs, Caps & Covers. The fluid path continues through Brake Lines, Hoses & Fittings to Brake Calipers & Brackets. Pick your machine in the Ride Finder at the top of the page.
FAQs on brake master cylinders
What does the bore size of a master cylinder mean?
The bore is the diameter of the piston inside, such as 5/8 inch. A smaller bore gives more line pressure for the same lever effort but needs more lever travel, and a larger bore gives the opposite. It should be matched to the caliper.
Should I rebuild or replace a master cylinder?
If the bore is smooth and only the seals are worn, a rebuild kit is often enough. If the bore is scored, pitted or corroded, or the body is damaged, replace the cylinder. Check the manual and the condition of the parts.
Is a clutch master cylinder the same as a brake master cylinder?
They work on the same principle but are listed as separate parts, and a clutch cylinder is not meant for the brakes. Use the one made for its job and follow the manual for the fluid your hydraulic clutch needs.
What brake fluid should I put in a master cylinder?
Use the DOT type printed on the cap or given in the manual. Do not mix silicone-based DOT 5 with glycol-based fluids such as DOT 3, 4 or 5.1. Brake fluid also damages paint, so wipe up spills quickly.
Why is my lever soft after fitting a new master cylinder?
Air is usually trapped in the system, so the brake must be bled. Make sure the cylinder is mounted level, the fluid is topped up and the fittings are sealed. If the lever is still soft after bleeding, check for leaks.
