Brake faults — consequence of vapour lock in hydraulic brakes What is the primary effect of vapour locking on vehicle brake performance?

Introduction / Context:
Vapour lock occurs when brake fluid boils in lines or calipers, creating compressible vapour pockets. Understanding the symptom is critical for safety and maintenance.

Given Data / Assumptions:

• Hydraulic service brakes using glycol ether or silicone-based fluid.
• High temperature from heavy braking or dragging brakes.
• Possible moisture contamination lowering fluid boiling point (wet boiling point).

Concept / Approach:
Hydraulic systems rely on near-incompressible fluid to transmit force. When vapour forms, it compresses under pedal force, absorbing stroke without generating sufficient line pressure at the calipers/wheel cylinders. The consequence can range from severe sponginess to complete loss of braking force.

Step-by-Step Solution:
Heat raises local fluid temperature above its boiling point → vapour bubbles form.Vapour compresses when the pedal is pressed → large pedal travel, little clamp force.In extreme cases, pedal goes to the floor with negligible braking → functional failure.

Verification / Alternative check:
Field reports and testing show dramatic loss of deceleration during fade with vapour lock; recovery may occur after cooling as vapour condenses.

Why Other Options Are Wrong:
More effective braking is impossible with compressible vapour present.

Delayed operation (option c) can occur early, but the dominant hazard is near-total loss under sustained pressure.

No effect is incorrect; the effect is severe.

Common Pitfalls:
Using old contaminated fluid (low wet boiling point); inadequate cooling; resting foot on pedal causing drag; failing to flush fluid at recommended intervals.

Final Answer:
brakes fail completely