Which component is most likely defective if a brake pedal slowly sinks to the floor with no loss of brake fluid?

Which component is most likely defective if a brake pedal slowly sinks to the floor with no loss of brake fluid?

• A. Brake Lines
• B. ABS Module
• C. Proportioning Valve
• D. Master Cylinder

Answer: (D)

When a brake pedal slowly sinks to the floor but there’s no visible brake fluid loss, the problem is a loss of hydraulic pressure from within the system. The master cylinder is responsible for generating and maintaining that pressure. If its internal seals or bore are worn, fluid can bypass the seals and flow back into the reservoir instead of staying pressurized when you hold the pedal. That internal leakage causes the pressure to bleed off, so the pedal slowly sinks even though you haven’t seen an external leak. The other components don’t fit this symptom as neatly: external leaks in brake lines would show fluid loss, the ABS module affects anti-lock braking behavior rather than steady pressure loss, and the proportioning valve changes balance of pressure to the wheels but doesn’t cause the pedal to sink from internal leakage. So the master cylinder is the most likely defective part.

When a brake pedal slowly sinks to the floor but there’s no visible brake fluid loss, the problem is a loss of hydraulic pressure from within the system. The master cylinder is responsible for generating and maintaining that pressure. If its internal seals or bore are worn, fluid can bypass the seals and flow back into the reservoir instead of staying pressurized when you hold the pedal. That internal leakage causes the pressure to bleed off, so the pedal slowly sinks even though you haven’t seen an external leak. The other components don’t fit this symptom as neatly: external leaks in brake lines would show fluid loss, the ABS module affects anti-lock braking behavior rather than steady pressure loss, and the proportioning valve changes balance of pressure to the wheels but doesn’t cause the pedal to sink from internal leakage. So the master cylinder is the most likely defective part.