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Question

Difficulty: MediumBraking Systems

A technician is preparing a replacement master cylinder for installation by performing a bench bleeding procedure to purge trapped air. What is the correct sequence of steps to properly complete this procedure?

  1. 1Mount the master cylinder securely and level in a bench vise.
  2. 2Thread bleeder tube fittings into the outlet ports and direct the tube ends into the reservoir chambers.
  3. 3Fill the reservoir with clean, specified brake fluid until the tube ends are fully submerged.
  4. 4Slowly stroke the piston inward with a smooth, blunt tool until no air bubbles emerge from the tubes.

Answer

The correct sequence begins by mounting the master cylinder level in a vise, followed by installing bleeder tubes routed into the reservoir, filling the reservoir so the tube ends are submerged in clean brake fluid, and finally slowly depressing the piston until all air bubbles cease escaping.
Bench bleeding is performed prior to vehicle installation to eliminate air trapped in the master cylinder bores. The cylinder must first be clamped level, followed by installing the bleed tubes, filling the reservoir to submerge the tube ends, and finally cycling the piston slowly until all air bubbles are expelled.

Step-by-Step Solution

1
Clamp the master cylinder level in a bench vise.
The master cylinder is held securely and horizontal.
Prevents spillage and ensures both the primary and secondary chambers fill evenly.
2
Attach the bleeder adapters to the outlet ports and route the plastic hoses into the reservoir.
A closed fluid loop is created between the hydraulic ports and reservoir.
Allows fluid and air to exit the ports and recirculate into the reservoir without admitting outside air.
3
Fill the reservoir with new brake fluid above the hose discharge ends.
The outlet hose ends are completely submerged in brake fluid.
Ensures only brake fluid—not atmospheric air—is drawn into the cylinder bore on the piston release stroke.
4
Depress the master cylinder piston slowly using a blunt tool and allow it to return fully, repeating until bubbles stop.
Trapped air is completely expelled from the internal piston passages.
Evacuating air prior to installation prevents a soft, spongy brake pedal and reduces on-vehicle bleeding time.

Key Concept

Master Cylinder Bench Bleeding Procedure
Estimated Time:1m 0s
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Topics

Braking Systems

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