Types of Divided Hydraulic Systems

A wheel’s braking ability depends on the load it is carrying; therefore, the type of vehicle is a major factor in determining how its system should be divided. A front-engine rear-wheel drive car has around 40% of its load on its rear wheels and 60% on its front wheels. Its braking system can therefore be divided in a vertical, or front–rear, split. This design puts the front wheels on a different system than the rear wheels. If one half of the system fails—the front or the rear—there is still enough separate braking capability left in the other half to stop the vehicle.

On a front-wheel drive vehicle, a load of about 20% on the rear wheels cannot provide enough braking force to stop the vehicle. Therefore, front-engine, front-wheel drive vehicles use a braking system split in a diagonal, or X, pattern. The left-hand front brake unit is connected to the right-hand rear unit, and the left-hand rear unit is connected to the right-hand front unit. If one system fails, a 50% braking capability is available in the other system.

An alternative arrangement for front-engine, front-wheel drive vehicles is an L split Figure 31-8. The front disc brake units have four piston calipers. One inner and one outer piston on each front caliper connect to the right-hand rear brake unit, and the other two pistons of each front caliper connect to the left-hand rear brake unit. As with the diagonal split system, if there is a failure of either half of the system, it still leaves 50% of the braking capability.

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FIGURE 31-8
Divided hydraulic systems. A. Vertical, or front–rear, split hydraulic system. B. Diagonal, or X, pattern hydraulic system. C. L-split hydraulic system.
 
Technician Tip
When diagnosing and servicing brakes, it is helpful to know how the hydraulic system is divided. If the vehicle is pulling to one side due to a leak in one half of the system, it will generally pull toward the side that is working in the front. Diagnosis and inspection of the other diagonal half will usually lead to the leak. In the same way, if air is trapped in one half of the hydraulic braking system, then bleeding the corresponding diagonal half will make it easier to remove the air.