Bleeding the Front Brake on a 1979 Honda CM400T
Shop parts for a 1979 Honda CM400T Motorcycle.The 1979 Honda CM400T uses a hydraulic disc brake at the front wheel and a mechanical drum brake at the rear. That distinction matters before you start: only the front system uses brake fluid and has a bleeder valve to service. The rear drum is operated by mechanical linkage and is adjusted, not bled. This guide walks you through refreshing the fluid and removing air from the CM400T’s front hydraulic brake so the lever feels firm and the brake responds predictably.
When the CM400T Front Brake Needs Bleeding
A spongy front lever is the classic sign that air is in the hydraulic 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 pressure. Dark, cloudy, or contaminated fluid in the master cylinder reservoir is another reason to service the brake. Bleeding is also required after any work that opened the hydraulic circuit, such as replacing the brake line, servicing the caliper, or repairing the master cylinder.
Even when the lever feels normal, 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 periodically as preventive maintenance rather than waiting for a soft lever. On a bike of this age, it is also worth checking that the front brake line and caliper are in good condition before spending time bleeding; a cracked line or leaking caliper will not be fixed by bleeding alone.
Tools and Supplies to Gather
- Fresh brake fluid of the type marked on the master cylinder reservoir cap, from a sealed container
- A box-end wrench that fits the front caliper bleeder valve
- A length of clear tubing that fits snugly over the bleeder valve
- A small 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 what this CM400T needs. Have a plan for the old fluid, too; it should go into the catch bottle and be disposed of properly rather than poured onto the ground.
Preparation Before Opening the Hydraulic System
Support the bike securely upright on a stand so it cannot tip while you are working at the lever and caliper. Turn the handlebars so the front master cylinder reservoir sits level before you remove the cap. Cover the painted surfaces, plastic bodywork, and instrument area with rags, because most brake fluids damage paint and plastic on contact. If a spill happens, wipe it up immediately and rinse the area with water. Keep the reservoir cap and diaphragm clean and set them where they will not pick up grit.
Locate the front caliper bleeder valve and confirm that the box-end wrench fits it properly. A box-end wrench is preferred because it grips the bleeder more securely than an open-end wrench. Slide the clear tubing over the bleeder and route the other end into the catch bottle, making sure the tubing stays submerged in a little old fluid if possible. That helps you see bubbles and prevents air from being drawn back in during the procedure.
Step-by-Step Front Brake Bleeding Procedure
- Remove the master cylinder reservoir cap and diaphragm. Use the syringe or turkey baster to remove most of the old fluid from the reservoir, but do not let the reservoir run dry and do not let the baster touch the bottom of the reservoir hard enough to disturb sediment.
- Refill the reservoir with fresh fluid of the type marked on the cap, leaving enough room to reinstall the diaphragm later. Keep the reservoir covered loosely with a rag to prevent splashes while you work.
- Confirm that the clear tubing is pushed fully onto the caliper bleeder valve and that the free end is in the catch bottle.
- Squeeze the front brake lever slowly and hold it against the grip. While you hold steady pressure, open the bleeder valve about a quarter turn with the box-end wrench.
- Watch fluid and air move through the clear tubing. Close the bleeder valve before you release the lever. Releasing the lever with the bleeder open can draw air back into the caliper.
- Release the lever slowly, then repeat the squeeze-hold-open-close-release sequence. Check the reservoir level frequently and top it off with fresh fluid so it never runs low enough to pull air into the master cylinder.
- Continue until the fluid coming through the tubing is clean and free of bubbles, and the lever begins to feel firm rather than spongy. Old fluid often looks darker or cloudier than the fresh fluid, so watch for that change as well as for bubbles.
- If your CM400T has been modified with a second front caliper, treat each caliper as its own bleed point and bleed them one at a time. As a general practice, start with the caliper farthest from the master cylinder, then move to the closer one.
- When the bleeding is complete, close the bleeder valve snugly with the box-end wrench. Tighten it firmly enough to seal, but do not overtighten it or you can damage the valve seat.
- Top off 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 drips.
Finishing and Final Checks
With the cap back on, pump the front lever a few times and inspect the reservoir level again. Look closely at the bleeder valve, the banjo bolt area, and the brake line connections for any sign of fluid. A dry fitting after several lever pumps is a good sign; a wet fitting needs attention before the bike is ridden. Clean the caliper and surrounding area with water if any fluid touched painted or plastic surfaces.
Spin the front wheel by hand to confirm the brake is not dragging. The wheel should turn freely with only the normal slight drag of the pads. If the wheel is difficult to turn or the brake feels applied, stop and investigate before riding.
Testing the CM400T Brake Before Riding
The front lever should feel firm and consistent, not spongy, and it should not slowly sink while you hold steady pressure. Test the brake at low speed in a safe area, such as an empty parking lot, before riding in traffic. Apply the brake gently and progressively, then more firmly, and confirm that it slows the bike smoothly without pulling or grabbing. Because the rear brake on this model is a mechanical drum, remember that it is adjusted separately and does not share fluid with the front system.
Troubleshooting and When to Visit a Shop
If the lever will not firm up after several bleeding attempts, or if it firms only after repeated pumping, air is likely still entering the system or trapped somewhere in the circuit. Persistent bubbles at the bleeder, fluid leaking from the master cylinder or caliper, or a lever that feels different every time you pull it are all reasons to stop and diagnose rather than keep riding. A brake line that bulges under pressure or a caliper that will not release also needs professional attention.
This CM400T is a conventional non-ABS, non-linked brake system, so there is no ABS unit or linked-brake sequence to complicate the job. If the bike has been modified with non-stock components, or if you cannot get a firm lever after a reasonable amount of bleeding, have a qualified technician inspect the system. Brakes are safety-critical, and a shop visit is far cheaper than a failure on the road.
Related Shopping Categories
Shop Brake Fluid for a 1979 Honda CM400T Motorcycle.
Shop Brake Lines for a 1979 Honda CM400T Motorcycle.
Shop Brake Pads for a 1979 Honda CM400T Motorcycle.
Shop Stands for a 1979 Honda CM400T Motorcycle.
Shop Brake Tools for a 1979 Honda CM400T 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.
