What type of brake fluid is DOT 4?
DOT 4 brake fluid is a glycol‑based hydraulic fluid with a higher boiling point than DOT 3, designed to handle greater heat in modern braking systems. It has a dry boiling point of at least 230°C (446°F) and a wet boiling point of at least 155°C (311°F); it is not silicone-based like DOT 5 and is commonly used in many passenger vehicles that demand higher heat resistance.
What DOT 4 brake fluid is
DOT 4 refers to a specification for a class of brake fluids that are glycol-based and formulated to resist boiling under higher brake temperatures. It is selected for its improved high‑temperature performance over DOT 3 and is hygroscopic, meaning it absorbs moisture over time, which can affect braking performance if not maintained properly.
Key properties
Below are the core properties that define DOT 4 and help distinguish it from other fluids:
- Base chemistry: glycol-based fluid (glycol ethers) with borate esters to raise the boiling point
- Boiling points: dry point at least 230°C (446°F); wet point at least 155°C (311°F)
- Hygroscopic nature: readily absorbs moisture from the air and the system, which lowers boiling point over time
- Compatibility: miscible with other glycol-based fluids such as DOT 3 and DOT 5.1; not compatible with silicone-based DOT 5
- Color and branding: varies by manufacturer; color is not a reliable indicator of performance
DOT 4's higher boiling points and glycol base make it a versatile choice for many vehicles, but it requires regular maintenance to manage moisture content and ensure compatibility with the braking system.
Boiling points and performance
DOT 4 has a dry boiling point of at least 230°C (446°F) and a wet boiling point of at least 155°C (311°F). The dry point applies to fresh fluid; the wet point reflects fluid that has absorbed moisture. Moisture in the fluid lowers the boiling point, increasing the risk of brake fade under heavy use. Regular replacement as recommended by vehicle manufacturers helps maintain performance.
Compatibility and safety considerations
DOT 4 is glycol-based and compatible with other glycol-based fluids such as DOT 3 and DOT 5.1. It is not compatible with silicone-based DOT 5; if converting from DOT 3 or DOT 5.1 to DOT 4, a full system flush is recommended to avoid mixing. Do not mix DOT 4 with non-glycol fluids or with mineral oil-based fluids.
Usage guidelines and maintenance
Where and how to use DOT 4, and what to watch for during maintenance:
- Where to use: recommended for most modern vehicles with ABS or brake assist that require higher heat resistance than DOT 3
- Mixing rules: can be topped up or replaced with other glycol-based fluids (DOT 3, 4, or 5.1) but should not mix with silicone DOT 5; fully flush when converting
- Replacement intervals: many manufacturers advise replacing every 2–3 years or sooner if moisture content is high; follow the vehicle manual
- Bleeding and flushing: after replacement or filling, bleed the brake lines to remove air
- Storage and handling: store in sealed containers away from moisture; avoid contamination; use clean equipment
Regularly using the correct DOT 4 fluid with proper maintenance helps ensure consistent braking performance and reduces corrosion risk.
Maintenance and replacement guidelines
Practical steps to maintain and replace DOT 4 fluid, including intervals and procedures:
- Replacement intervals: flush and replace DOT 4 per manufacturer guidance, typically every 2–3 years
- Moisture testing: use a brake fluid refractometer or test strips; moisture above ~3% compromises boiling point
- Bleeding: bleed the system after replacement to remove trapped air
- Disposal: dispose of used fluid according to local regulations; do not pour down drains
- Safety: wear gloves, avoid skin contact, prevent spills on paint
Adhering to these maintenance steps helps preserve braking reliability and system longevity.
Summary
DOT 4 is a glycol-based brake fluid with higher boiling points than DOT 3, offering improved heat resistance for modern braking systems. It is compatible with other glycol-based fluids (such as DOT 3 and DOT 5.1) but not with silicone-based DOT 5, and it requires regular moisture management and system flushing to maintain optimal performance. Always follow the vehicle manufacturer's specifications for fluid type and replacement intervals.
