How To Bleed 1979 Honda CM400A - Hondamatic Motorcycle Brakes

Bleeding the Front Brake on a 1979 Honda CM400A Hondamatic

The 1979 Honda CM400A Hondamatic uses a hydraulic disc brake at the front and a mechanical drum brake at the rear. That configuration matters before you start: the front brake is the only hydraulic circuit that needs bleeding, while the rear drum is adjusted with its linkage and does not use brake fluid. This bike also predates ABS and linked brake systems, so the job stays focused on the front master cylinder, hose, and caliper. If your lever has gone soft, the fluid looks dark, or you have just opened the front hydraulic system for hose or caliper work, this procedure will walk you through a complete flush and air bleed.

When the CM400A Needs a Brake Bleed

A spongy front lever is the classic sign that air is in the system. You may also notice the lever pulling closer to the grip than it used to, or a lever that feels firm at first and then slowly sinks while you hold it. Dark, cloudy, or contaminated fluid in the reservoir is another reason to bleed. Any time the hydraulic system has been opened—new brake lines, caliper service, or master cylinder work—air can enter and must be removed before the bike is ridden.

Even without symptoms, brake fluid absorbs moisture from the air over time. Moisture lowers the fluid’s boiling point, which can cause brake fade under hard or repeated use. Many riders refresh the fluid on a regular schedule as preventive maintenance. On a bike as old as the CM400A, it is also worth checking that the reservoir cap is sealing properly and that the hose and caliper are not weeping fluid before you begin.

Tools and 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 bleeder valve on the front caliper
• A length of clear tubing that fits snugly over the bleeder valve
• A clean catch bottle for the old fluid
• Rags or shop towels, plus a syringe or turkey baster for removing old fluid from the reservoir
• A screwdriver or driver that fits 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 do not mix fluid types. The reservoir cap marking is the authority for this bike. Have all supplies laid out before you open the reservoir so you are not searching for a tool while the system is exposed.

Set Up the Bike and Protect the Finish

Support the CM400A securely upright on a stand so it cannot tip while you work. Turn the handlebars until the front reservoir sits level; if the reservoir is tilted, you cannot judge the fluid level accurately and it is easier to spill. Cover the painted surfaces, plastics, and the instrument area with rags before opening anything. Most brake fluids damage paint and plastic on contact, so wipe up any spill immediately and rinse the area with water if fluid reaches a painted or plastic surface.

Clean the area around the reservoir cap before removing it. Dirt that falls into the reservoir can contaminate the system and cause problems later. Once the cap is off, do not leave the reservoir open any longer than necessary.

Step-by-Step Front Brake Bleed

1. Open the front master cylinder reservoir and remove most of the old fluid with a syringe or turkey baster. Leave enough fluid in the bottom so you do not introduce air through the master cylinder outlet, but remove the dark, contaminated fluid before refilling.
2. Refill the reservoir with fresh fluid of the type marked on the cap. Keep the bottle capped when you are not pouring, and do not reuse fluid that has been sitting open.
3. Attach the clear tubing to the front caliper bleeder valve and route the other end into the catch bottle. The tubing should fit snugly so air cannot be drawn in around the bleeder.
4. Squeeze and hold the front brake lever. Keep steady pressure on it while you open the bleeder valve about a quarter turn with the box-end wrench. Fluid and air will travel through the clear tube.
5. Close the bleeder valve before you release the lever. If you release the lever while the bleeder is open, air can be drawn back into the caliper. This is the most important rhythm in the entire job: squeeze, hold, open, close, release.
6. Repeat the squeeze-open-close-release cycle, checking the reservoir level every few cycles. Top off with fresh fluid before the level drops low enough to draw air into the master cylinder. Never let the reservoir run dry.
7. Continue until the fluid coming through the clear tube is fresh, clean, and free of bubbles. Watch the tube closely; small bubbles can be hard to see against dark fluid, so use a clean catch bottle and good light.
8. If you are ever bleeding a front end with two calipers, treat each caliper as its own bleed point and do them one at a time. As a general practice, start with the caliper farthest from the master cylinder. The CM400A’s front caliper is the bleed point for this job.

Finishing the Bleed and Checking for Leaks

When the fluid runs clean and bubble-free, close the bleeder valve snugly with the box-end wrench. Tighten it firmly enough to seal, but do not overtighten it; the bleeder is small and can be damaged by excessive force. Fill the reservoir to the correct level marking, reinstall the diaphragm and cap, and wipe away any spilled fluid. Check the bleeder valve and the banjo fittings at the master cylinder and caliper for weeping or wetness. A dry fitting after several lever pulls is what you want to see.

Test 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 feels soft or the wheel drags, stop and recheck your work rather than assuming it will improve on the road.

If the Lever Will Not Firm Up

If the lever will not firm up after several bleeding attempts, or if it firms only after pumping, air is still entering or trapped somewhere. Persistent bubbles, fluid leaking from the master cylinder or caliper, or a lever that never feels consistent are reasons to stop and have a shop inspect the system. Because this CM400A has neither ABS nor linked brakes, you do not have to worry about an ABS unit trapping air or a linked system’s extra bleed points, but a worn master cylinder seal or a damaged hose can still make a home bleed unsuccessful. When in doubt, let a professional pressure-test and repair the system before you ride.

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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.