Brake fluid specifications: What is the principal difference between DOT 3 and DOT 4 glycol-based brake fluids as defined by performance standards?

Introduction / Context:

Hydraulic brake fluids must resist vaporization under severe braking to prevent pedal fade. Department of Transportation (DOT) classifications specify minimum dry/wet boiling points and other properties. Choosing the correct fluid maintains braking reliability, especially in high-temperature conditions.

Given Data / Assumptions:

• Glycol-ether based fluids (DOT 3 and DOT 4), both hygroscopic.
• Dry boiling point (fresh fluid) and wet boiling point (after absorbing moisture) are key metrics.
• System elastomer compatibility assumed.

Concept / Approach:

DOT 4 is formulated (often with borate esters) to deliver higher minimum dry and wet boiling points than DOT 3. Both absorb moisture from the atmosphere over time, lowering boiling points, but DOT 4 starts higher and maintains better high-temperature performance. Freezing resistance and silicone content distinctions apply to DOT 5 (silicone, non-hygroscopic), not to DOT 3/4.

Step-by-Step Solution:

Verification / Alternative check:

Standards (e.g., FMVSS No. 116) list minimum boiling points showing DOT 4 > DOT 3 for both dry and wet ratings.

Why Other Options Are Wrong:

• Lower boiling point / same boiling point: contrary to the specification tables.
• More resistant to freezing: not the defining difference.
• Contains silicone: describes DOT 5, not DOT 3/4.

Common Pitfalls:

• Mixing DOT 5 (silicone) with glycol fluids—generally not compatible.
• Assuming higher class always suitable; check manufacturer recommendations and ABS/ESP compatibility.

Final Answer:

DOT 4 fluid has a higher boiling point than DOT 3 fluid