Brake Fluid – Types, Tips
And Why You Need To Bleed

There are two common kinds of brake fluids used today: polyglycol based and silicone based.

Glycol based fluids are know as DOT 3, DOT 4 and DOT 5.1. These fluids are hygroscopic, which means that they attract and absorb water. DOT 5.1 is a synthetic fluid that resists moisture absorption and provides better performance in harsh conditions. DOT 3 and 4 types are typically used in all production vehicles, although some manufacturers have started to use DOT 5.1.

Silicone based fluids are known as DOT 5. Silicone is actually compressible, (causing a spongy pedal) which is one of the reasons it isn’t used in production braking systems. It also tends to foam at high pumping rates, which can happen when the ABS system activates. This foaming will create air bubbles in the system - which will cause a spongy brake pedal.

Silicone fluids are non-hygroscopic, so they don’t absorb water. This type of fluid is commonly used in military vehicles, heavy equipment, and classic/antique cars. Silicone is a good brake fluid for vehicles that won’t be driven much, or where braking performance isn’t a priority. Silicone fluid can be up to 10 times more expensive that glycol based fluids.

Dry and Wet Boiling Points
All brake fluid has a dry boiling point and a wet boiling point. The dry boiling point is the temperature that fresh, uncontaminated fluid will boil. This is the number that is important when choosing a fluid, and the higher temperatures are better. The wet boiling point is the temperature that the fluid will boil when contaminated with 3% water by volume.

Fluid compatibility
DOT 3, DOT 4 and DOT 5.1 are all glycol based fluids and are compatible with one another. When flushing the system or bleeding the brakes, always use the recommended DOT fluid for your vehicle or one with a higher number.

In other words, don’t add DOT 3 to a system that uses DOT 4. You will lower the boiling point of the system below the recommended level. Check your owner’s manual for the type of fluid used in your vehicle.

Don’t use racing fluid unless you have a race car. Although these fluids are able to withstand severe use and have high boiling points, they are extremely hygroscopic. Their wet boiling points can be lower than normal fluid.

Silicone fluid is NOT compatible with glycol based fluid and should never be mixed.

Water in the fluid
Water in the braking system is a problem whatever type of brake fluid is in your vehicle. When water mixes with glycol based fluids, the boiling point of the mixture is lowered, increasing the chances of the fluid overheating. How does water get into the system in the first place? Through the master cylinder cap, caliper seals and even through the rubber brake hoses.

Silicone fluids don’t mix with water, so the water sinks to the bottom of the braking system (usually to a caliper or wheel cylinder) because it is heavier. Not only will the water corrode brake components from the inside, the boiling point of the fluid will drop to 212°F (the boiling point of water) due to a high concentration of water in the caliper. This water will vaporize when overheated, causing air bubbles to form in the fluid. Any water in the brake fluid must be bled out.

Air in the fluid
Air in the fluid can come from overheated brakes, improper fluid type or a leak in the system. Air trapped in the fluid is dangerous! They last thing you want to worry about is having to pump the brakes during a panic stop! Any air in the lines will need to be bled out after any leaks are fixed.

Overheated brakes
The fluid must be able to withstand the high heat generated by the brake system. If the fluid overheats it will boil and create air bubbles. Since air is compressible, the braking performance will be decreased and result in a spongy pedal. If this happens, the brakes must be bled to remove the air bubbles.

Solutions for overheating brakes include:

• Bleed brakes and replace fresh fluid
• Replace pads with performance type with higher fade resistance
• Use crossed drilled rotors which will dissipate heat faster
• Switch to a brake fluid with a higher boiling point.

Why you need to bleed
It’s recommended that brake fluid be replaced every 2 years based on normal driving for maximum braking effectiveness. Any water absorbed in the fluid will be flushed out with a brake fluid change. Bleeding brakes is the only way to flush out the whole system of air and water from the hoses and lines.

How To Bleed Brakes Instructions

Brake fluid tips:

• Don’t leave the master cylinder fluid reservoir cap off for very long. Glycol based fluids will absorb water from the air and lower boiling temperatures.

• As the brake pads wear, the fluid level drops in the reservoir. Before new pads can be installed, the caliper piston must be retracted. As the piston is pushed back into the caliper, the fluid level will rise back up. Check the reservoir level after the repair is completed and add more fluid if necessary.

• Glycol based fluids are corrosive and harmful to paint. If spilled, wash thoroughly with water. Silicone fluid is not harmful to paint.

• It’s recommended to never use fluid from a container that has previously been opened. Old fluid should never be reused.

• If the master cylinder reservoir requires frequent filling to keep it at the proper level, then the system should be checked for leaks.

• Buy fluid in small quantities to minimize waste.

• Follow the service recommendation for inspecting and flushing the brake fluid in your vehicle.

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