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Basic Hydraulic Theory For Vehicle Brake System

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Brake systems use hydraulic fluid in a closed system to transmit motion. The hydraulic brake system is governed by physical laws that makes it efficient at transmitting both motion and force. Blaise Pascal discovered the scientific laws governing the behavior of liquids under pressure. Pascal’s Law states that pressure applied anywhere to an enclosed body of fluid is transmitted equally to all parts of the fluid. In other words, 100 psi generated at the master cylinder is the same at each wheel cylinder as well as anywhere within a static system.

A feature of hydraulic theory can be seen in the illustration below which demonstrates the pressure in the master cylinder is transmitted equally to all wheel cylinders.

hydraulic theory pascal's law


Another important distinction to make is that liquids cannot be compressed, whereas, air is compressible. A hydraulic system must be free of air in order to function properly. Pedal travel will increase as air in the system is compressed.

liquid cannot be campressed


Fluid pressure is indicated in pounds per square inch (psi). It is determined by dividing the input force applied to a piston by the area of the piston. (force/area = pressure in psi) If a force of 100 pounds is applied to a master cylinder piston, an area of 2 square inches, the resulting pressure will be 50 psi. This pressure is transmitted to all parts of the fluid in the container equally.

force / area = psi
100 / 2 = 50 psi

In the series of examples below we are examining working force and transfer of motion based on different working piston diameters. In each example, piston A is the same diameter (1") and the same 100 lb. Input force is applied. When the force is applied to piston A, piston B has 100 psi of output force and travels an equal distance to piston A.

By contrast piston C will have an output force of twice that of piston A because piston C has twice the area. In addition, piston C transfers only half the distance of piston A.

Yet another contrast is piston D which is half the area of piston A. The system pressure is the same as the two previous examples but since piston D is half the area of piston A, the pressure is half the apply pressure and the motion transfer is twice that of piston A.

working force and transfer of motion


Hydraulic brakes deliver equal braking force to all wheels with a minimum of transmission loss. Hydraulic brakes have a wide design flexibility because braking force can be changed merely by changing the diameter of the master cylinder and wheel cylinders.

Brake Fluid:

Brake fluid is specifically designed to be compatible with its environment of high heat, high pressure and moving parts. Standards for brake fluid have been established by the Society of Automotive Engineers (SAE) and the Department of Transportation (DOT). Requirements of a fluid used in automotive brake applications must include the following:

remain viscous.
have a high boiling point.
act as lubricant for moving parts.

The Federal Motor Vehicle Safety Standard (FMVSS) states that by law, brake fluid must be compatible regardless of manufacturer. Fluids are not necessarily identical however, any DOT approved brake fluid can be mixed with any other approved brake fluid without damaging chemical reactions. Although the fluid may not always blend together into a single solution, it does not effect the properties of liquid under pressure.

Brake Fluid Types:

Two types of brake fluid are used in automotive brake applications, each having specific attributes and drawbacks. Polyglycol is clear to amber in color and is the most common brake fluid used in the industry. It is a solvent and will immediately begin to dissolve paint. Flush the area with water if brake fluid is spilled on paint.

One of the negative characteristics of polyglycol is that it is hygroscopic, that is, it has a propensity to attract water. Water can be absorbed through rubber hoses and past seals and past the vent in the master cylinder reservoir cap. Moisture in the hydraulic circuit reduces the boiling point of the fluid and causes it to vaporize. In addition, moisture causes metal parts to corrode resulting in leakage and /or frozen wheel cylinder pistons.

Extra caution should be taken with containers of brake fluid because it absorbs moisture from the air when the container is opened. Do not leave the container uncapped and close it tightly.

Silicone is purple in color. It is not hygroscopic and therefore has virtually no rust and corrosion problems. It has a high boiling point and can be used in higher heat applications. It will not harm paint when it comes in contact with it.

Silicone has a greater affinity for air than polyglycol. Because the air remains suspended in the fluid it is more difficult to bleed air from the hydraulic system.

DOT Grades:

There are three grades of brake fluid which are determined by Federal Motor Vehicle Safety Standard 116. Fluid grades are rated by the minimum boiling point for both pure fluid (dry) and water contaminated fluid (wet):

DOT 3 − Polyglycol
   minimum boiling point − 401°F dry, 284 °F wet
   blends with DOT 4

DOT 4 − Polyglycol
   minimum boiling point − 446 °F dry, 311 °F wet
   blends with DOT3

DOT 5 − Silicone
   minimum boiling point − 500 °F dry, 356 °F wet
   compatible by law with DOT 3 and 4 but will not blend with them.


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