How to Rebuild Honda TRX400EX Front Master Cylinder 

Rebuilding the front brake master cylinder on a Honda ATV takes a fair amount of time to complete, but is otherwise a pretty straightforward job.

Watch the video above and follow the steps below to rebuild the front master brake cylinder on a Honda TRX 400EX.

Tools and Parts Needed – Honda TRX400 Front Master Brake Cylinder Rebuild

Tools

 

Parts

NOTE: Brake fluid damages plastics and paintwork, and even the smallest bit of dirt can compromise the braking system’s performance. Have plenty of rags or shop towels ready for brake fluid spills, and keep everything perfectly clean.

Honda TRX400EX front brake master cylinder rebuild

Removing Honda TRX400EX Front Brake Master Cylinder

Step 1. Remove the master cylinder cap, the diaphragm and diaphragm plate beneath it. Use an impact driver to remove the two cap screws without stripping the heads.

Honda TRX 400 front master cylinder removal

Step 2. Remove the dust cap from the bleed screw on the front brake caliper and connect a length of 3/16” hose. Run the hose down to a container to catch the old brake fluid.

Honda TRX 400 front brake caliper bleed

Step 3. Drain the brake system by pumping the front brake lever a couple of times, holding the lever in and releasing the 10mm bleed screw. Tighten the screw, then pump the lever again, hold the lever and release the screw once more. Repeat this process until all the brake fluid is out of the system.

Step 4. Pull back the brake lever cover and remove the 10mm locknut holding the pivot bolt from the underside of the lever.

Honda ATV front brake master cylinder removal

Step 5. Remove the pivot bolt that holds the brake lever, followed by the lever itself. 

Step 6. Disconnect the two wires from the brake light switch beneath the master cylinder.

Honda TRX400 ATV front brake master cylinder

Step 7. Remove the banjo bolt that connects the brake fluid line to the master cylinder. 

NOTE: Don’t lose the two banjo bolt washers.

Step 8. Remove the clamp that holds the master cylinder to the handlebar, starting with the clamp’s lower 8mm bolt first. Twist the clamp around the handlebar so the upper 8mm bolt clears the throttle cable, then remove the upper bolt.

Rebuilding Honda TRX400EX Front Brake Master Cylinder

Step 1. Remove the rubber boot from the master cylinder.

Step 2. Remove the circlip from inside the master cylinder by freeing it from its groove with circlip pliers, then using a pick tool to pry it out.

Step 3. Remove the piston and the spring from the master cylinder.

Honda TRX400 master cylinder piston spring

Step 4. Thoroughly clean the master cylinder with contact/brake cleaner and blow it out with compressed air.

Step 5. Inspect the bore in the master cylinder where the piston sits. It should be perfectly smooth with no signs of corrosion, scratches or pitting. If the bore shows even the slightest wear, replace the master cylinder. 

Honda TRX 400 front brake master cylinder bore

Step 6. Smear the new piston and spring from the master cylinder kit in brake fluid to lubricate the rubber seals. Next, slide the piston and spring into the master cylinder bore.

Step 7. Install the new circlip from the master cylinder kit by getting it inside the bore with circlip pliers and pushing it down the bore with a pick tool until it snaps into position.

Honda TRX400 ATV front brake master cylinder rebuild

Step 8. Install the new rubber boot from the master cylinder kit.

Reinstalling Honda TRX400EX Front Brake Master Cylinder

Step 1. Reinstall the master cylinder clamp by first installing the upper clamp bolt, then twisting the clamp so the master cylinder is upright and installing the lower clamp bolt.

Step 2. Reinstall the banjo bolt that connects the brake fluid line to the master cylinder, followed by the washers on both sides of the banjo bolt.

Step 3. Reconnect the two wires to the brake light switch beneath the master cylinder. 

NOTE: The polarity of the wires won’t affect the switch, so you can reconnect them either way around.

Step 4. Reinstall the brake lever, the pivot bolt and the pivot bolt locknut, and pull the brake lever cover back into place.

Honda TRX 400 EX front brake lever

Bleeding Honda TRX400EX Front Brake Master Cylinder

Step 1. Fill the master cylinder reservoir to about three quarters full with DOT-4 brake fluid.

Step 2. Pump the brake lever a few times and with the lever held in, release the 10mm bleed screw on the brake caliper and then tighten it again.

Step 3. Repeat this process 3-4 times, then top off the master cylinder reservoir with brake fluid. 

NOTE: Don’t let the reservoir run dry while you’re bleeding the brakes. Doing so introduces air into the system and you’ll have to start all over again.

Step 4. Continue the bleeding process until there’s a steady stream of brake fluid with no air bubbles coming from the bleed valve. Fully tighten the bleed valve and replace its dust cap.

Honda TRX400 EX brake bleeding

Step 5. Repeat the brake bleeding process on the other front brake caliper. 

NOTE: You need to bleed both calipers because the master cylinder serves both sides of the braking system.

Step 6. Check the rubber diaphragm that sits beneath the master cylinder’s cap for splits or tears, and replace it if necessary.

Honda TRX 400 front brake master cylinder diaphragm

Step 7. Top off the master cylinder reservoir with brake fluid, then reinstall the diaphragm, diaphragm plate and cap. 

NOTE: Don’t over-tighten the cap screws because the master cylinder is made of aluminum and you don’t want to strip the screw threads.

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