A technician is troubleshooting a brake system on a vehicle that uses DOT 5 brake fluid. The client reports a spongy pedal at times, especially after the vehicle has been sitting for a while, but it improves slightly after driving. The technician notices the fluid in the reservoir appears slightly cloudy. What is the MOST likely cause of this condition?
- AOverheating of the brake fluid during heavy braking.
- BContamination of the DOT 5 fluid with petroleum products.
- CDegradation of internal master cylinder seals.
- DAir entrapment due to the non-hygroscopic nature of DOT 5 fluid.
Show answer & explanationAnswer & explanation
Correct answer: D. Air entrapment due to the non-hygroscopic nature of DOT 5 fluid.
DOT 5 (silicone-based) fluid is non-hygroscopic, meaning it does not absorb water. However, it is more prone to aerating and trapping air bubbles, which can cause a spongy pedal. The cloudy appearance is often due to these trapped air bubbles, especially after sitting, and they can be difficult to bleed out. The spongy pedal improving after driving suggests the air bubbles might be settling or moving, but still present.
Why the other options are wrong
- A. DOT 5 has a very high boiling point and is less prone to overheating than glycol-based fluids, so this is unlikely to be the primary cause of a spongy pedal in DOT 5.
- B. Contamination with petroleum products would cause severe seal damage and potential total brake failure, not typically an intermittent spongy pedal with cloudy fluid.
- C. Degradation of master cylinder seals would cause a pedal that sinks slowly, not typically an intermittent spongy pedal with cloudy fluid, unless it's drawing in air which then gets trapped.
DOT 5 Fluid Aeration
DOT 5 (silicone-based) brake fluid is non-hygroscopic, but it has a tendency to aerate, meaning it can trap air bubbles more easily than glycol-based fluids. These trapped air bubbles can cause a spongy brake pedal and make the fluid appear cloudy, requiring careful bleeding procedures.
- Silicone-based.
- Non-hygroscopic.
- Prone to air entrapment/aeration.
- Causes spongy pedal.
- Fluid appears cloudy.
Memory trick: DOT 5's cloudy, air's aloud, making the pedal a spongy shroud!