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Brake Fluid

 

Automotive brake fluid is formulated to meet standards developed by the Department of Transportation (DOT) and the Society of Automotive Engineers. The Department of Transportation assigns a number indicating the fluid rating: DOT3, DOT4, DOT5. The higher the number the higher the boiling point.

Brake fluid is either silicon or glycol based. Never mix the two and always use the type of fluid recommended for the vehicle your working on. Another thing to remember is that glycol based brake fluid will damage paint and should be handled accordingly.

As mentioned earlier the higher the DOT number the higher the boiling point. This is important because heat generated from friction is transferred from the braking system to the brake fluid. This heat can cause the brake fluid to boil producing vapor. A hydraulic braking system works because fluids are not compressible. The problem with brake fluid is when it boils it converts to a vapor. This vapor is compressible and will result in reduced braking performance or worse. A brake system flush with the proper brake fluid is necessary to remedy this condition.

Most of today's vehicles use DOT3 glycol (Polyalkylene Glycol Ether) type brake fluid. Silicon based fluids (DOT5) are usually found in special applications like show cars because it doesn't damage painted surfaces like it's glycol counterpart. All brake fluids must meet DOT standards for corrosion protection, lubrication, boiling point and more.