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Module 2 - Hydraulic System Components (Page 3 of 5)

Hydraulic System Components

Conbination Valves - Valves are used

Valves are used in the hydraulic brake system to control the flow of fluid and the pressures applied throughout the system.

Why do we need to control the pressures? Because ...

  1. Disc brakes require higher operating pressures than drum brakes.
  2. Drum brakes must overcome spring tension and higher shoe-to-drum clearances so they must be applied first before the disc brakes are applied
  3. Brake balance - pickup trucks that are empty require less rear brake application than if they are hauling a truck full of gravel.

These valves may include:

1. Pressure differential valve

The pressure differential valve senses a leak or loss of pressure in each of the independent circuits. See previous page for more details. When a leak is detected, a valve actuates a switch and turns on a red brake warning lamp to warn the driver of a brake problem.

2. Metering valve

The metering valve provides balanced braking on vehicle equipped with front disc and rear drum brakes. The metering valve delays the operation of the disc brakes until the rear drums have started to brake / operate. This is needed because disc brakes are fast-acting while drum brakes must overcome spring tension and clearances before operating.

3. Proportioning valve

During braking, the vehicle weight will shift forward. The front brakes must provide the majority of braking action. The proportioning valve regulates / limits pressure to the rear brakes to prevent the rear wheel from locking up.

4. Pressure control valve

A pressure control valve (also called a height sensing or load sensing valve) regulates the amount of braking pressure to the rear wheels. This helps to prevent wheel lockup while providing optimum brake pressure for safe braking.

The operation of a pressure control valve is very similar to a proportioning valve. This valve is particularly important on pickup trucks where the load / weight of the vehicle may vary considerably.

5. Residual pressure valve

The residual pressure valve is used in drum brake systems to hold low pressures in the rear wheel cylinder brake circuits. This low pressure helps to keep the wheel cylinder cup seals in contact witrh the cylinder walls. This prevents the seal lips from moving away from the walls and allowing air to enter the system.

Sometimes the above valves are combined into a single COMBINATION VALVE

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Module 2 (Page 3 of 5)