How To Bench Bleed Master Cylinder

How to Bench Bleed a Master Cylinder: A Comprehensive Guide

Are you experiencing spongy brakes or a lack of braking power? A faulty master cylinder could be the culprit. Before replacing it entirely, consider bench bleeding your master cylinder. This process removes air trapped within the cylinder, restoring proper brake function. This guide will walk you through the process, ensuring you do it safely and effectively.

What is Bench Bleeding?

Bench bleeding is a preventative maintenance step, and is crucial before installing a new or rebuilt master cylinder. It involves purging air from the cylinder before it's installed on your vehicle. This avoids the hassle and potential damage associated with bleeding the brakes on the car itself after installation. This method significantly simplifies the process and ensures optimal braking performance from the start.

Tools You'll Need

Before you begin, gather these essential tools:

• New Master Cylinder: Obviously, you'll need a new or rebuilt master cylinder to bench bleed.
• Brake Fluid: Use the correct type of brake fluid specified for your vehicle. Never mix different types of brake fluid.
• Clear Tubing: Two pieces of clear tubing, long enough to reach a container for collecting the fluid.
• Container: A suitable container (e.g., jar, bowl) to collect the used brake fluid.
• Pliers or Vise Grips (Optional): To securely hold the master cylinder.
• Shop Towels or Rags: To clean up any spills.
• Funnel: A small funnel to make filling the master cylinder easier.
• Gloves: Protect your hands from brake fluid.

Step-by-Step Bench Bleeding Guide

Step 1: Preparation

1. Cleanliness is Key: Ensure your workspace is clean and free of debris. Brake fluid can damage paint and other surfaces.
2. Protect Your Hands: Wear gloves to prevent skin irritation from brake fluid.

Step 2: Assemble Your System

1. Attach the Tubing: Attach the clear tubing securely to the outlet ports on the master cylinder. You'll likely have two ports, one for front brakes and one for rear. Make sure the tubing is a snug fit.
2. Submerge the Tubing: Submerge the other ends of the tubing into the container filled with a small amount of brake fluid. This helps prevent air from being sucked back into the system.

Step 3: Filling the Master Cylinder

1. Fill 'er Up: Using a funnel, slowly fill the master cylinder reservoir with fresh brake fluid until it's almost full. Avoid overfilling.
2. Prime the Pump: This is a key step to getting things started. Carefully and gently operate the master cylinder's push rod (typically done by pushing and pulling the piston). This helps to initiate fluid flow and begin purging the air.

Step 4: The Bleeding Process

1. Repeated Pumping: Continue pumping the push rod of the master cylinder in a smooth, controlled motion. Observe the fluid in the tubing carefully. You should see bubbles rise to the surface, indicating the air is being expelled.
2. Fluid Flow: Keep pumping until a steady stream of fluid, free of air bubbles, flows through both tubes. Maintain the fluid level in the reservoir.
3. Persistence is Key: This can take some time, depending on the amount of air present.

Step 5: Final Checks

1. Inspect Fluid: Ensure the fluid is consistently free of bubbles for several pump cycles in each tube.
2. Top Off: Top off the master cylinder reservoir with fresh brake fluid.
3. Carefully Remove Tubing: Once you are satisfied with the bleeding process, remove the tubes carefully from the ports.

Step 6: Installation

Now that your master cylinder is bench bled, it's ready for installation in your vehicle. Follow your vehicle's specific instructions for this process. Remember, after installation, a final brake bleed is recommended at the wheels to remove any residual air.

Troubleshooting Tips

• Stubborn Air Bubbles: If you're having trouble removing stubborn air bubbles, try gently tapping or rotating the master cylinder.
• Fluid Leaks: Check for leaks around the master cylinder during the process.
• Incorrect Fluid: Using the wrong type of brake fluid can damage your brake system.

By carefully following these steps, you can effectively bench bleed your master cylinder and restore your braking system to optimal performance. Remember, safety should always be your top priority when working on your vehicle’s braking system. If you're unsure about any part of this process, consult a qualified mechanic.