Automotive brakes and vapour lock — identify the operating condition that most commonly causes vapour locking in a hydraulic brake system, leading to a soft or fading pedal after repeated use.

Introduction / Context:
Vapour locking in a hydraulic brake system is a dangerous condition in which portions of the brake fluid boil and form compressible vapour pockets. Because hydraulic systems rely on incompressible fluid to transmit force, the presence of vapour leads to spongy pedal feel, long pedal travel, and a drastic loss of braking efficiency. The question focuses on the primary operating condition that triggers this failure mode.

Given Data / Assumptions:

• Conventional hydraulic service brakes with brake fluid having a finite dry and wet boiling point.
• Typical scenarios include frequent or prolonged braking, high ambient and rotor temperatures, and potential moisture absorption that lowers boiling point.
• No special racing brake fluids or exotic cooling systems are assumed.

Concept / Approach:

Brake fluid temperature rises when friction work at the pads and discs or drums converts kinetic energy into heat. Repeated heavy applications, like downhill descents or stop-and-go driving with aggressive braking, elevate caliper and fluid temperatures. Once local fluid temperature exceeds the effective boiling point (which is reduced by absorbed moisture), vapour forms. These bubbles compress under pedal force, so less line pressure reaches calipers, producing fade and possible temporary brake failure until the vapour condenses again.

Step-by-Step Solution:

Verification / Alternative check:

Service literature advises using high boiling point DOT-rated fluids, periodic fluid replacement to reduce moisture content, and proper brake cooling to mitigate vapour lock, confirming that overheating is the root cause.

Why Other Options Are Wrong:

Overcooling during high speed driving would increase safety margin rather than induce boiling. Keeping a vehicle unused may invite corrosion or seal issues but does not directly create vapour lock. Engine speed on a downhill road is irrelevant unless braking is frequent and heavy; the overheating itself is the trigger.

Common Pitfalls:

Confusing pad fade from binder outgassing with fluid vapour lock; assuming only racing conditions cause boiling; overlooking the effect of moisture that lowers the wet boiling point.

Final Answer:

overheating of the fluid due to frequent brake application