Brake Fluid Characteristics
Brake fluid is a speciality blended liquid that provides a means of transmitting hydraulic pressure from the master cylinder to the wheel cylinders and calipers. Federal laws require that brake fluid must meet certain standards and specifications before it is sold. The most important characteristics that any brake fluid must possess are:
- Viscosity - It should be free flowing at all temperatures.
- High Boiling Point - It should remain in a liquid state at the highest operating temperatures that might be encountered.
- Non-corrosive - It should not attack rubber or metal parts and it should inhibit various types of corrosion and act as an anti-rust agent.
- Water-tolerant - It should be able to absorb and retain moisture that collects in the system.
- Lubricating Ability - It should lubricate pistons and cups to reduce wear and internal friction.
- Low Freezing Point - It should remain fluid and flow easily at low operating temperatures.
| Symptom | Cause | Solution |
|---|---|---|
| Low or Spongy Brake Pedal | Air in Hydraulic System Leaking Master Cylinder Secondary Cup Faulty Residual Pressure Valve |
Bleed Hydraulic System Rebuild/Replace Master Cylinder Rebuild/Replace Master Cylinder |
| Excessive Brake Pedal Travel | Leaking Master Cylinder Primary Cup | Rebuild/Replace Master Cylinder |
| Excessive Brake Pedal Effort | Seized or Sticking Master Cylinder Piston | Rebuild/Replace Master Cylinder |
| Dragging Brakes | Blocked Compensation Port Faulty Residual Pressure Valve Swollen Master Cylinder Cups |
Rebuild/Replace Master Cylinder Rebuild/Replace Master Cylinder Rebuild/Replace Master Cylinder |
The U.S. Department of Transportation has set specifications for three types of brake fluid for automotive use: DOT 3, DOT 4 and DOT 5. Each type has a specified color.
DOT 3 (or SAE J1703) and DOT 4 are amber to clear in color. The major difference between the two formulations is that DOT 4 has a higher wet boiling point and absorbs moisture more slowly than DOT 3 fluid.
DOT 5 brake fluid is silicone-based, purple in color, and has a higher boiling point than either DOT 3 or 4. Both DOT 3 and 4 are inter changable/mixable. However, DOT 5 should not be mixed with either DOT 3 or 4.