DOT 3 versus DOT 4 brake fluid
Brake fluid DOT 3 and DOT 4 are made with glycol-ether and boil lowering additives and anti-corrosive additives. Glycol-ether is used because it has a high boiling point and is compatible with the rubber components used in an automotive braking system.
• DOT 3 fluid is approximately 80% glycol ether base.
• DOT 4 is made with 50 to 65% glycol ether base with 20-30 % Borate Ester to resist boiling.
• DOT 3 boiling point is 401°F dry and 284°F wet.
• DOT 4 boiling point is 450°F dry and 311°F wet
Even though DOT 4 has a higher boiling point than DOT 3, it has one major drawback; DOT 4 absorbs moisture more readily (about 20% faster) than DOT 3. That’s why DOT 3 is preferred by many carmakers.
DOT 4 fluid can be used in a vehicle where DOT 3 is specified, but DOT 3 should never be used on a vehicle where DOT 4 is specified because the DOT 3 boiling point will be lower than the brake system’s minimum specifications.
©, 2021 Rick Muscoplat