How To Bleed 1981 Honda CM400A - Hondamatic Motorcycle Brakes

Bleeding the Front Brake on a 1981 Honda CM400A Hondamatic

The 1981 Honda CM400A Hondamatic uses a hydraulic front disc brake and a mechanical rear drum brake. That distinction matters before you start: the front brake has fluid, a master cylinder, a caliper, and a bleeder valve, while the rear drum is operated by mechanical linkage and does not use brake fluid at all. The rear drum is adjusted, not bled. This guide therefore focuses on refreshing the fluid and removing air from the front hydraulic brake on the CM400A. The system is straightforward and non-ABS, with no linked-brake sequence to follow, so a careful caliper bleed is the correct approach.

When the CM400A Front Brake Needs Bleeding

A spongy front lever, a lever that pulls closer to the grip than it used to, or a lever that feels inconsistent is the classic sign that air is in the hydraulic system. Dark, cloudy, or contaminated fluid in the reservoir is another reason to service the brake. 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, brake fluid absorbs moisture over time, which lowers its boiling point and can contribute to fade under hard use, so many riders refresh it on a regular schedule. If the rear brake feels weak, remember that the CM400A rear drum is mechanical; check its adjustment rather than looking for a bleeder on the rear wheel.

Tools & Supplies for the Job

• Fresh brake fluid of the type marked on the master cylinder reservoir cap, from a sealed container
• A box-end wrench that fits the caliper bleeder valve
• Clear tubing that fits snugly over the bleeder valve
• A small catch bottle for the old fluid
• Rags or shop towels, plus a way to rinse spilled fluid with water
• 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 sealed container, and confirm the required fluid type on the reservoir cap before opening anything. Do not guess the fluid type or substitute an unknown fluid.

Preparing the Bike and Protecting the Finish

Support the CM400A securely upright on a stand so the front end is stable. Turn the handlebars until the front master cylinder reservoir sits level; if the reservoir is tilted, it is easy to spill fluid or draw air at the cap opening. Cover the painted surfaces, plastic bodywork, and instrument area with rags before you remove the cap. Most brake fluids damage paint and plastic on contact, so wipe up any spill immediately and rinse the area with water. Work in a well-ventilated area and keep the reservoir cap and diaphragm clean once removed.

Step-by-Step Front Brake Bleed

1. Remove the reservoir cap and diaphragm. Use the syringe or turkey baster to draw most of the old fluid out of the reservoir, but do not let the reservoir run dry and do not let the baster touch the bottom and stir up sediment.
2. Refill the reservoir with fresh fluid of the type marked on the cap, leaving it at the proper level for now. Keep the cap nearby and covered to keep dirt out.
3. Push the clear tubing onto the caliper bleeder valve and route the other end into the catch bottle. A clear tube lets you watch for air bubbles and confirm when clean fluid arrives.
4. Squeeze the front brake lever slowly and hold it under pressure. Do not pump rapidly with the bleeder open.
5. With the lever held, open the bleeder valve about a quarter turn with the box-end wrench. Fluid and air will travel down the tube. Close the bleeder before you release the lever.
6. Release the lever only after the bleeder is closed. Check the reservoir level and top it off before the next cycle. Never let the reservoir get low enough to draw air into the master cylinder.
7. Repeat the squeeze-hold-open-close-release cycle until fresh, clean fluid comes through the tube with no visible bubbles. Keep topping off the reservoir throughout the process.
8. On this front hydraulic brake, work at the caliper bleeder valve. If a modified CM400A has two front calipers, bleed each one in turn, starting with the caliper farthest from the master cylinder.
9. When the fluid runs clear and bubble-free, close the bleeder snugly. Do not overtighten it; snug is enough to seal without damaging the valve.

Finishing the Bleed

Fill the reservoir to the correct level marking, then reinstall the diaphragm and cap. Wipe any spilled fluid from the reservoir, caliper, hose, and surrounding parts, and rinse with water if fluid reached paint or plastic. Check for leaks at the bleeder valve and at the banjo fittings on the master cylinder and caliper. Pump the front lever a few times to restore normal feel, then inspect the lever and the caliper area again for weeping fluid.

Testing 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 before riding normally. If the lever is still soft, or if it firms up only after several pumps, there is likely still air in the system or a leak that needs attention.

Troubleshooting and When to Stop

If the lever will not firm up after several careful bleeding attempts, if persistent bubbles keep appearing, or if fluid is leaking from the master cylinder or caliper, stop and have the bike inspected. A lever that firms only after pumping also points to remaining air or a seal problem. This CM400A is not an ABS or linked-brake model, so a standard caliper bleed is appropriate here; on a bike with ABS or linked brakes, air trapped in those units can require dealer-level equipment, and the job should go to a shop. Do not ride with a brake that feels soft, inconsistent, or unable to hold pressure.

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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.