What is the difference between Brake fluid DOT 3 or DOT 4?
07-09-2001, 06:47 AM
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What is the difference between Brake fluid DOT 3 or DOT 4?
I was topping off some fluids and the brake resevoir stated to use DOT 4. I was shopping for brake fluid and they sold both types but what is the difference? Is it like motor oil with the different viscosity grades? Thickness? hmm.
07-09-2001, 10:21 AM
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DOT 4 has a higher boiling point, mainly.
It also has different water-absorption properties, etc...if you use DOT 3 when it says DOT 4, the car will be unsafe to drive, but you can use 4 in place of 3 without any problems.
07-09-2001, 01:37 PM
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The ABC's of Brake Fluid!
When you depress your brake pedal a rod connected to the pedal exerts pressure on a piston which forces brake fluid from the brake fluid resevoir into your master brake cylinder. The brake pedal pressure is transmitted and amplified, from the master cylinder, via brake lines to your calipers or wheel cylinders which force grouped pairs of brake pads into contact with either the brake rotors or drums which bring you and your vehicle to a safe stop.
Friction is created when the brake pads or shoes press against the rotors or drums. This friction is what causes your car to stop when you apply the brakes. The heat that builds up in your drums and rotors (in certain cases the surface temperatures of which can exceed 1333 degrees fahrenheit) also causes the brake fluid to heat up. This is why DOT #3 and DOT #4 brake fluids usually have a boiling point which surpasses 401 degrees fahrenheit (the higher the DOT number, the higher the fluid's minimum boiling point, i.e. DOT #3 is 401 deg. F., DOT #4 is 446 deg. F., and DOT #5 is
500 deg. F). Over time, though, the heat generated from braking causes the chemical composition of the brake fluid to change. Gasses are boiled out of the brake fluid and since these gasses are more compressible than the brake fluid they came from, you will notice a soft or spongy brake pedal feel.
DOT #3 and DOT #4 brake fluids are made from Polyalkylene Glycol Ether which, by nature, has a very strong attraction to water (brake fluid is hygroscopic). Water contaminating brake fluid makes its boiling point drop which causes bubbles to form in the brake fluid (imagine sprinkling water into a frying pan full of hot oil). Bubbles and water are more compressible than brake fluid which makes it more difficult for the master cylinder to compress the brake fluid and in turn, compress the calipers and brake drums. A sign of this may be a low, soft, or spongy brake pedal feel (caused by the the contaminating bubbles and water compressing before the brake fluid does).
DOT #5 brake fluid, on the other hand, is formulated from silicone. While silicone is not hygroscopic and has a higher boiling point than DOT#3 and DOT #4 brake fluid, any water that gets into the brake system will sink to the lowest point of that system (water being heavier than silicone) and may potentially find its way to the calipers, boil at 212 degrees fahrenheit and cause a vapor lock which could cause brake failure. Additionally, DOT #5 brake fluid, being a synthetic, is much more expensive than DOT #3 and DOT #4 fluids, hence Tire Kingdom uses DOT #5 brake fluid only at the request of the customer.
Water is bad for your brakes. " As little as 3% moisture contamination can reduce the boiling point of brake fluid by as much as 25%. By the time a new car is only 12 months old, it's brake fluid contains about 2% water. After 18 months, the water content is approaching 3%. After several years of service it is not uncommon to find brake fluid containing 7 to 8 % water. This can lead to a condition called 'fluid boil'. In this condition the driver runs out of pedal stroke without actuating the brakes. Brake heat boils the fluid into vapor so that the pedal stroke is used up to compress gas. With a sufficient vapor accumulation, the master cylinder can run out of stroke before the brakes are applied. Often this condition disappears before being checked, since the vapor converts back to a fluid when it cools." Water in the brake system can also cause internal corrosion (rusting) of the metal and deterioration of the rubber parts of the brake system from the master cylinder through the brake lines and seals down to the wheel cylinders and caliper pistons. Replacing these parts can be an expensive proposition, especially if the vehicle is equipped with ABS (anti-lock brakes).
How does water get into the brake system? Moisture enters through microscopic pores in the rubber brake hoses and seals, cracks in the rubber brake hoses, leaks in the brake lines, and when the brake fluid resevoir is opened (it does not take much for brake fluid to absorb moisture, especially in a place like Florida where there tends to be a great deal of humidity in the air).
How can you tell when your brake fluid is dirty or contaminated? The color of the brake fluid itself, offers a clue. New, clean brake fluid usually has a golden color (unless it is DOT #5 which may be purple or yellow), while dirty old brake fluid is brown or if heavily contaminated, black. A brake fluid moisture tester can also be used to gauge the purity of the fluid in your brake system.
Most major vehicle manufacturers recommend having your brake fluid replaced about every 2 years (more often in severe driving conditions). Draining and replacing your brake fluid helps to remove contaminants (such as water and rust) and to lubricate the metal and rubber parts of your brake system (like the master cylinder, brake lines and seals, valves, wheel cylinders and caliper pistons) which may help them last longer.
Brake fluid is a technically sophisticated and usage specific chemical. No other fluid can perform the functions it is designed for:
Brake fluid must not boil under high temperatures.
Brake fluid must retain its viscosity at temperatures at or below freezing.
Brake fluid must not readily compress.
Brake fluid must flow freely through small gauge orifices.
Brake fluid must not corrode or react with materials in the brake system.
Brake fluid must be able to lubricate the moving, metallic, and rubber parts of the brake system.
Brake fluid must remain chemically stable for extended periods of time.
Brake fluid must be miscible (mixable or compatible) with other glycol based products.
Brake fluid must not decompose or form gum or sludge in the brake system.
Friction is created when the brake pads or shoes press against the rotors or drums. This friction is what causes your car to stop when you apply the brakes. The heat that builds up in your drums and rotors (in certain cases the surface temperatures of which can exceed 1333 degrees fahrenheit) also causes the brake fluid to heat up. This is why DOT #3 and DOT #4 brake fluids usually have a boiling point which surpasses 401 degrees fahrenheit (the higher the DOT number, the higher the fluid's minimum boiling point, i.e. DOT #3 is 401 deg. F., DOT #4 is 446 deg. F., and DOT #5 is
500 deg. F). Over time, though, the heat generated from braking causes the chemical composition of the brake fluid to change. Gasses are boiled out of the brake fluid and since these gasses are more compressible than the brake fluid they came from, you will notice a soft or spongy brake pedal feel.
DOT #3 and DOT #4 brake fluids are made from Polyalkylene Glycol Ether which, by nature, has a very strong attraction to water (brake fluid is hygroscopic). Water contaminating brake fluid makes its boiling point drop which causes bubbles to form in the brake fluid (imagine sprinkling water into a frying pan full of hot oil). Bubbles and water are more compressible than brake fluid which makes it more difficult for the master cylinder to compress the brake fluid and in turn, compress the calipers and brake drums. A sign of this may be a low, soft, or spongy brake pedal feel (caused by the the contaminating bubbles and water compressing before the brake fluid does).
DOT #5 brake fluid, on the other hand, is formulated from silicone. While silicone is not hygroscopic and has a higher boiling point than DOT#3 and DOT #4 brake fluid, any water that gets into the brake system will sink to the lowest point of that system (water being heavier than silicone) and may potentially find its way to the calipers, boil at 212 degrees fahrenheit and cause a vapor lock which could cause brake failure. Additionally, DOT #5 brake fluid, being a synthetic, is much more expensive than DOT #3 and DOT #4 fluids, hence Tire Kingdom uses DOT #5 brake fluid only at the request of the customer.
Water is bad for your brakes. " As little as 3% moisture contamination can reduce the boiling point of brake fluid by as much as 25%. By the time a new car is only 12 months old, it's brake fluid contains about 2% water. After 18 months, the water content is approaching 3%. After several years of service it is not uncommon to find brake fluid containing 7 to 8 % water. This can lead to a condition called 'fluid boil'. In this condition the driver runs out of pedal stroke without actuating the brakes. Brake heat boils the fluid into vapor so that the pedal stroke is used up to compress gas. With a sufficient vapor accumulation, the master cylinder can run out of stroke before the brakes are applied. Often this condition disappears before being checked, since the vapor converts back to a fluid when it cools." Water in the brake system can also cause internal corrosion (rusting) of the metal and deterioration of the rubber parts of the brake system from the master cylinder through the brake lines and seals down to the wheel cylinders and caliper pistons. Replacing these parts can be an expensive proposition, especially if the vehicle is equipped with ABS (anti-lock brakes).
How does water get into the brake system? Moisture enters through microscopic pores in the rubber brake hoses and seals, cracks in the rubber brake hoses, leaks in the brake lines, and when the brake fluid resevoir is opened (it does not take much for brake fluid to absorb moisture, especially in a place like Florida where there tends to be a great deal of humidity in the air).
How can you tell when your brake fluid is dirty or contaminated? The color of the brake fluid itself, offers a clue. New, clean brake fluid usually has a golden color (unless it is DOT #5 which may be purple or yellow), while dirty old brake fluid is brown or if heavily contaminated, black. A brake fluid moisture tester can also be used to gauge the purity of the fluid in your brake system.
Most major vehicle manufacturers recommend having your brake fluid replaced about every 2 years (more often in severe driving conditions). Draining and replacing your brake fluid helps to remove contaminants (such as water and rust) and to lubricate the metal and rubber parts of your brake system (like the master cylinder, brake lines and seals, valves, wheel cylinders and caliper pistons) which may help them last longer.
Brake fluid is a technically sophisticated and usage specific chemical. No other fluid can perform the functions it is designed for:
Brake fluid must not boil under high temperatures.
Brake fluid must retain its viscosity at temperatures at or below freezing.
Brake fluid must not readily compress.
Brake fluid must flow freely through small gauge orifices.
Brake fluid must not corrode or react with materials in the brake system.
Brake fluid must be able to lubricate the moving, metallic, and rubber parts of the brake system.
Brake fluid must remain chemically stable for extended periods of time.
Brake fluid must be miscible (mixable or compatible) with other glycol based products.
Brake fluid must not decompose or form gum or sludge in the brake system.
