How To Bleed 1973 Kawasaki S2 - Mach 2 Motorcycle Brakes

Bleeding the Front Brake on a 1973 Kawasaki S2 Mach 2

The 1973 Kawasaki S2 Mach 2 uses a hydraulic disc brake at the front and a mechanical drum brake at the rear. That difference matters before you touch a wrench: only the front system contains brake fluid and has a bleeder to service. The rear drum is operated mechanically and is adjusted rather than bled. This guide therefore focuses on refreshing the fluid and removing air from the S2’s front hydraulic brake. In stock form, the S2 has no ABS and no linked brake system, so the job is a straightforward manual bleed at the front caliper. If your bike has been modified, confirm what is actually fitted before opening the reservoir.

Why and When the S2 Front Brake Needs Bleeding

Brake fluid absorbs moisture from the air over time. That moisture lowers the fluid’s boiling point, which can cause brake fade under hard or repeated use, and it can also encourage corrosion inside the system. On a 1973 Kawasaki S2 Mach 2, the fluid in the reservoir may look dark or cloudy long before the lever feels wrong. A spongy lever, a lever that pulls closer to the grip than it used to, or a lever that slowly sinks under steady pressure are all signs that air may be in the system. Bleeding is also required after any work that opened the hydraulic circuit, such as replacing a brake line, servicing the caliper, or repairing the master cylinder. Even without symptoms, many riders refresh brake fluid on a regular schedule because moisture buildup is invisible from the outside.

Tools and Supplies to Gather First

• Fresh brake fluid of the type marked on the master cylinder reservoir cap, from a sealed container. Do not substitute a different type.
• A box-end wrench that fits the front caliper bleeder valve.
• Clear tubing that fits snugly over the bleeder nipple, plus a small catch bottle.
• Rags or shop towels, and a syringe or turkey baster for removing old fluid from the reservoir.
• The correct screwdriver or driver for the reservoir cap screws.
• Optional: a one-way check-valve bleeder or vacuum bleeder to make one-person bleeding easier.

Use only fluid from a freshly opened container, and discard what is left after the job. Brake fluid that has been sitting open can already have absorbed moisture. Keep the reservoir cap and diaphragm clean and set them on a clean rag, not on the garage floor.

Preparing the Bike and Protecting Surfaces

Support the S2 securely upright on a stand so it cannot tip while you are working at the handlebar. Turn the handlebars until the front master cylinder reservoir sits level; otherwise fluid can spill as soon as the cap comes off. Cover the painted surfaces, plastics, instrument area, and any bodywork below the reservoir with rags or shop towels. Most brake fluids damage paint and plastic on contact, so wipe up any spill immediately and rinse the area with water. Work slowly around the reservoir, and keep the cap off only while you are actively adding or removing fluid.

Bleeding the Front Brake, Step by Step

1. Clean the area around the reservoir cap, then remove the cap and diaphragm. Use a syringe or turkey baster to remove most of the old fluid from the reservoir without letting it run dry. Leave a little fluid at the bottom so you do not push air into the master cylinder.
2. Refill the reservoir with fresh fluid of the type marked on the cap, up to the level marking. Keep the cap and diaphragm nearby, but do not seal the reservoir yet.
3. Push the clear tubing onto the front caliper bleeder valve so it fits snugly, and route the other end into the catch bottle. Make sure the tube stays submerged or angled so you can see bubbles clearly.
4. Squeeze the front brake lever slowly and hold it against the grip. Do not release the lever while the bleeder is open.
5. Open the bleeder valve about a quarter turn with the box-end wrench. Fluid and any trapped air will flow through the tube. Watch the tube for bubbles and for the color of the fluid.
6. Close the bleeder snugly before you release the lever. Then release the lever slowly. Closing first prevents air from being drawn back into the caliper.
7. Repeat the squeeze-hold-open-close-release sequence. Check the reservoir after every few cycles and top it off with fresh fluid. Never let the reservoir run low enough to draw air into the master cylinder.
8. Continue until fresh, clean fluid with no bubbles comes through the tube. If you are flushing old fluid, keep going until the old fluid has been displaced. If your bike has two front calipers, bleed each one separately, starting with the caliper farthest from the master cylinder.
9. When the bleed is complete, close the bleeder snugly without overtightening. Replace the diaphragm and reservoir cap.

Finishing Up and Checking for Leaks

Fill the reservoir to the correct level marking, then reinstall the diaphragm and cap. Wipe away any spilled fluid and rinse the area with water if fluid touched paint or plastic. Check the bleeder valve and the banjo fittings at the master cylinder and caliper for weeping or wetness. Operate the front lever several times and look again for leaks. A bleeder that is snug but not overtorqued should not seep; if it does, stop and correct the problem before riding.

Testing the Brake Before You Ride

The front lever should feel firm and consistent, not spongy, and it should not slowly sink while you hold steady pressure. Spin the front wheel to confirm the brake is not dragging. Test the brake at low speed in a safe area, such as a empty parking lot, before riding normally. If the lever still feels soft or the brake does not engage predictably, do not ride the bike. Air remaining in the system is a safety issue, not something to sort out on the road.

Troubleshooting and When to Take It to a Shop

If the lever will not firm up after several bleeding attempts, or if it firms only after you pump it, air is still trapped somewhere. Recheck that the reservoir never ran dry, that the bleeder was closed before the lever was released, and that the tubing was snug on the bleeder. Persistent bubbles can also mean air is being drawn past the bleeder threads or that the master cylinder is not sealing correctly. Fluid leaking from the master cylinder or caliper is another reason to stop. This S2 has no ABS or linked brake system in stock form, so those procedures do not apply. If your bike has been modified with ABS or linked brakes and air has entered the unit or its lines, the system may need a dealer-level procedure or diagnostic tool; take it to a shop rather than guessing.

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Shop Brake Tools for a 1973 Kawasaki S2 - Mach 2 Motorcycle.

Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.