Classic Motorcycle Build

How To Rebuild Harley Master Cylinder

If your motorcycle master cylinder isn't holding pressure when you pull the brake lever, the internal seals may be worn or damaged.

Harley Sportster master cylinder rebuild

Other things that would cause no brake pressure are low brake fluid and leaking calipers, so check those two things out before blaming the master cylinder! If you find that it is bad, consider rebuilding the old one as an alternative to buying a new one.

The bike in this article is a 1976 Harley Sportster. The master cylinder is actually a chrome aftermarket replacement, but rebuild kits work just the same as with original Harley units.

how to rebuild Harley master cylinder

Master Cylinder Rebuild Kit

This master cylinder rebuild kit replaces Harley # 45063-72. It fits 3/4" bore master cylinders from many 1972 through 1981 Harleys, including FL, FX, and Sportster models.

master cylinder rebuild kit for Harley FL FX XL models 1972-1981

NOTE: The rebuild kit does not include a M/C cover gasket. If yours is damaged, you can buy one here.


Getting Started

Although you can rebuild a master cylinder while it's still mounted to the handlebars, it's a whole lot easier when it's sitting in front of you on a work bench.

Removing the brake fluid from the reservoir was done with a two-ounce (60 ml) syringe.

rebuild Harley master cylinder

Once the reservoir was empty, the front brake line was disconnected, and the four Allen bolts were removed. Disconnect the two thin wires from the stoplight switch.

rebuild Harley master cylinder

Moving over to the workbench, disassembly can begin.

rebuild Harley master cylinder

To remove the hand brake lever, remove the retaining ring (E-Clip) that holds the pivot pin.

rebuild Harley master cylinder

I'm using a small flat-blade screwdriver to remove the E-Clip, then pushing the pin through to release the lever.

rebuild Harley master cylinder

Brake lever removed.

Remove Master Cylinder Piston

You now have access to the retaining ring that holds in the piston assembly. Remove it with snap-ring pliers.

rebuild Harley master cylinder

It's a little tough to get to the snap-ring because it's recessed.

Switching from straight tips to right-angle tips on the snap ring pliers made this task easy.

swap straight tips for right-angle tips on snap-ring pliers

Pull out the piston, O-ring, piston cup, spring cup, and piston return spring.

rebuild motorcycle brake master cylinder

Clean and inspect all parts.

how to rebuild Harley master cylinder

If you find a little corrosion inside the bore, you can try cleaning it with some Emory cloth, but if there's a lot, it's best to replace the master cylinder.



rebuild Harley Sportster master cylinder

Assembly is basically reverse of disassembly.

rebuild Harley master cylinder

Before installing, lubricate new parts with DOT 5 brake fluid.

how to rebuild Harley master cylinder

Install the spring and piston into the cylinder bore, making sure they go in the correct way. If you're not sure of the sequence, refer to the illustration.

remove e-clip Harley master cylinder

Getting the brake lever back in was a little tricky. It helped putting the master cylinder in a vice between two soft jaw pads so that both your hands are free.

Test Master Cylinder

Here's an easy way to test a motorcycle master cylinder before hooking everything back up. Hold a finger over the outlet, then pump it several times. As you let your finger go, you'll hear the air escape.


Bleed Master Cylinder

There's several ways to bleed the master cylinder. I took a piece of 1/4" x 8" brake line from an auto parts store, cut one end off, and bent it so that it fits into the reservoir.

bleed front Harley master cylinder

Fill reservoir with DOT 5 brake fluid. Pump the hand lever slowly until air bubbles stop appearing in the fluid. On this one, it took about 10 or 15 pulls for the air bubbles to stop. The steel line was then removed and the front brake hose was attached.

Sportster caliper rebuild

Bleeding the front brake caliper.

As an alternative to rebuilding a Harley master cylinder you can buy a new one here.

motorcycle brake master cylinder


CAUTION: Working on brake systems is not something to be taken lightly. If you don't feel that you are 100% capable of doing it correctly, leave it to a professional.


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Harley Pie-Slice Caliper Rebuild
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Replace Fork Seals
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Read: 1976 Sportster Project