The two general types of fluid pressure are known as open channel flow and closed conduits. Fluids under open conditions, such as air in the atmosphere or water in the ocean, are referred to as being hydrostatic. Closed conduit fluids can be either hydrostatic or hydrodynamic, and these are the conditions that are used to perform various types of work in automobiles. Two common areas that fluid pressure can be used are the brake and power steering systems. These systems both use fluid pressure to magnify force applied by the driver to steer or stop the vehicle.
Hydrostatic pressure exists in any fluid that is not moving. Open bodies, such as oceans and the atmosphere, are typically referred to as hydrostatic even though there are localized movements within these fluids. In closed systems, such as brake and power steering lines, the fluids can be either static or dynamic. At rest, these systems are considered to be hydrostatic. When the power steering pump operates or the brake master cylinder is activated, they are governed by fluid dynamics.
Some automobiles use what is known as manual steering. The driver provides all the necessary power to turn the wheels in these systems, so it can be very difficult to maneuver the vehicle. Power steering systems use pressurized fluid to do much of the work for the driver, which can make a vehicle much easier to turn. These systems consist of a pump, a rack and pinion unit or steering gearbox, and lines that run between them. The pump is typically driven by the engine via the crank pulley, though some vehicles use cam driven pumps or other setups.
After passing through a power steering pump, fluid is sent through a pressure line to a steering gear. Valves within the gear are then used to direct the pressurized flow within the unit, which results in easier turning of the steering wheel. The fluid then passes back to the pump through a low pressure line and is subsequently recirculated.
Brake systems that use fluid pressure operate in a slightly different manner. These rely on a system of lines and cylinders that are sealed from the outside environment. When the driver depresses the brake pedal, the master cylinder creates fluid pressure within the lines. This fluid pressure then activates slave cylinders located at each wheel.
If air is allowed to enter one of these systems, it will not function properly because air and brake fluid compress differently. The system can also fail if a leak develops because fluid pressure requires a closed conduit to build up. It is also possible for a brake caliper, the slave cylinder in drum brakes, to become stuck. In this case, the brake caliper pressure may not release until the system is opened manually by loosening a bleeder screw.