Types of Brake Fluids

Written by rick chaney
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Types of Brake Fluids
Brake fluid is a very important part of any brake system. (old engine image by John Sfondilias from Fotolia.com)

Brake fluid is a very important part of the brake system. Brake systems need to be able to function through all types of conditions. It is crucial to choose the right brake fluid for the type of automobile and the climate that the car is exposed to. The Department of Transportation (DOT) has taken these factors into consideration in classifying the different brake fluid types. Each brake fluid type will have formulation or chemical differences and different dry and wet boiling points.

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DOT 3 Brake Fluid

The conventional brake fluid that most cars and light trucks use is the DOT 3 brake fluid. This poly glycol base product has a dry minimum boiling point of 205 degrees C, with the wet minimum boiling point at 140 degrees C. DOT 3 brake fluid can safely be mixed with DOT 4 and DOT 5.1 brake fluids. AA1Car.com recommends changing DOT 3 brake fluid once a year in humid climates or every 18 months in dryer climates. The boiling point of this brake fluid is reduced as it absorbs moisture; this reduces its performance, making the brake pedal feel spongy. DOT 3 brake fluid can damage a vehicle's paint if it comes in contact with the paint.

DOT 4 Brake Fluid

DOT 4 brake fluid is used in larger cars and full-sized trucks. This brake fluid is also used for ABS brake systems, towing, high speed braking and for vehicles in high altitude areas. DOT 4 brake fluid is a poly glycol base product that has a dry minimum boiling point at 230 degrees C, with the wet minimum boiling point at 155 degrees C. The DOT 4 brake fluid can safely be mixed with DOT 3 and DOT 5.1 brake fluids. DOT 4 brake fluid does not absorb moisture as quickly as DOT 3. However, the boiling point is reduced at a much faster rate. AA1Car.com recommends changing this type of brake fluid once in a year in humid climates and every 18 months in dryer climates. DOT 4 brake fluid also will damage the vehicle's paint if there is contact.

DOT 5 Brake fluid

DOT 5 brake fluid is mostly used in antique or collector automobiles that are not driven for long periods of time. DOT 5 brake fluid is not suitable for ABS systems. This brake fluid is silicone based and has a dry minimum boiling point of 260 degrees C, with the wet minimum boiling point at 180 degrees C. DOT 5 brake fluid cannot be mixed with any other type of brake fluid. This type of brake fluid does not absorb water. However, it does absorb air, making the pedal feel soft or spongy. DOT 5 brake fluid is purple in colour, and it will not harm a vehicle's paint. Special care needs to be taken if switching to a DOT 5 brake fluid, according to the Thunderbird and Cougar Club of America; consult an automotive expert if you want to make this switch.

DOT 5.1 Brake Fluid

DOT 5.1 brake fluid has a higher boiling point compared to DOT 3 and DOT 4. It is used in fleets, delivery trucks and other heavy-duty vehicles. This type of brake fluid is also popular in race cars. DOT 5.1 brake fluid is a poly glycol base product that has a dry minimum boiling point at 260 degrees C, with the wet boiling point at 180 degrees C. This brake fluid type can be safely mixed with DOT 3 and DOT 4 brake fluids. DOT 5.1 brake fluid absorbs moisture that will reduce the boiling point. In humid climates, DOT 5.1 brake fluid should be changed once a year; change the fluid every 18 months in dryer climates. This type of brake fluid can damage a vehicle's paint.

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