Master Cylinder Rebuild: 1973 Honda CB500

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This is what our CB500 brake master cylinder looked like after removing it from the bike. Removal is straightforward, requiring only that you disconnect the brake line from the master cylinder after either draining the fluid out through the brake caliper bleed screw or by sucking the fluid out with a vacuum pump.
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Here's our master cylinder and its component parts, but before removing its internal piston, seal and related parts. The rubber diaphragm at top left goes between the cap and the body fo the master cylinder and keeps brake fluid from sloshing around. The bolt and two sealing washers are for the brake line that attaches to the end of the cylinder.
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Once you've removed the lever from the master cylinder you'll be able to see the end of the piston. A small rubber boat keeps dirt and water out of the cylinder bore and is held in place by a very small wire clip. Since you'll be replacing both items, just push a small screwdriver through the boot and pry it out. It will usually slip out complete from under the wire clip, and it's much easier to get the clip out once the boot's been removed.
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Once the boot and its clip have been removed you can remove the piston and seals, after having removed the small snap ring (third part from the left, after the spring clip and the boot) that holds the piston, seals, spring and check valve in place.
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Once you have the master apart it's time to give it a thorough cleaning with brake parts cleaner followed by a quick hone. We made a flexible cylinder hone using a piece of 5/16-inch fuel line with a piece of fine-grade emery cloth wrapped around it and glued at the ends.
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We then took our homemade hone and slipped it onto a screw driver fitting and chucked it into a cordless drill. With the drill set on slow, and using brake fluid to lubricate the inside of the master cylinder, we spun the hone and, moving it slowly but regularly up and down the length of the bore. This ensures a thorough and even cleaning of the bore. Hone it until the entire bore shows a clean metal surface.
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This photo shows the piston and seals loosely assembled. I should have taken the piston assembly and rotated it 180 degrees to show how it goes into the master cylinder body.
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Here are all the new internal pieces for our brake master cylinder, set against a manual for comparison. There are two seals (visible to the right of the piston), one of which has to be slid over the piston and into a recess. We found it easiest starting at the left side of the piston as shown, working it over the shaft and then over the raised boss in the center and into its recess. The seal is very strong, but be careful so you don't tear the seal.
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Almost done! Here's the piston fully inserted into the brake master cylinder bore. Note the metal washer that follows it. The snap ring goes on top of this and into a groove in the cylinder wall. This is what holds it all together. Once that's in, you can slip the protective boot in place, followed by the wire clip, which you can push down and against the lip of the boot with a small screwdriver.
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Here, we're just starting to put the master cylinder together. We've already placed the check valve and spring inside, followed by the cupped primary seal. Use the piston to push the seal down into the bore. It can be a bit difficult to get it to go in straight, but after a few tries you'll get it.
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Once the piston's been installed, it's a simple matter of re-attaching the brake handle, dropping the rubber diaphragm back in place and replacing the cap. Depending upon where you got your parts you'll get a new set of sealing washers for the brake line. Once you have that hooked up it's just a matter of bleeding out the system by filling it the master with fresh brake fluid and pumping it through the bleeder valve at the caliper until fresh fluid comes out with no sign of air. That's it!

This rebuild came about as part of Project Café, the 1973 Honda CB500 Four we’ve been morphing into a street café racer with help from BikeBandit.com. We covered rebuilding the brake caliper in the September/October 2009 issue, a project you can also find here on our website.

Although this particular rebuild deals with the master cylinder from our ’73 CB500, Honda used basically the same master cylinder (and caliper) on just about every disc brake-equipped Honda starting with the CB750 in 1969 up until about 1978, when they started making significant changes to their hardware, although the GL series used a different style master cylinder and caliper. As with any project like this, we highly recommend having a manual on hand to help guide you through the process and to give you exploded images of parts so you can better understand how they all go together. Pay particular attention to cleanliness. You don’t want to contaminate the brake system with any dirt, grease, water … anything. Brake parts cleaner is cheap: use plenty of it as you work along and allow plenty of time for parts to air dry before moving on. Use brake fluid as a lubricant when you’re honing the cylinder bore and when you’re putting internal master cylinder parts together and into the master cylinder bore.

The image gallery will take you through our rebuild step-by-step. And don’t forget, if there are issues we’ve neglected to point out or tips you can add to our rebuild, just use the “Comments” feature to add your input to the discussion. – Richard Backus

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