with rear-wheel antilock brakes should step firmly with care. and if they feel the wheel locking, they should release some pressure.

Expect noise and vibration in the brake pedal when antitock brakes are in use. The mechanical noise or pulsation of antilock brakes when in use might catch an operator by surprise; however, these sensations tell you that the system is working.

Remember that you can steer while braking with a four-wheel antilock brake system. Steering is not always instinctive in an emergency. But steer out of danger while braking with antilock brakes. And remember that while you have steering capability, your vehicle may not turn as quickly white braking on a slippery road, as it would on dry pavement.

The rear-wheel antitock brakes typically found on light-duty trucks provide vehicle stability but do not give you the steering capability of four-wheel antilock brakes.

Anti lock brakes can often stop more quickly than conventional brakes but they can't overcome the law of physics. Antilock brakes function well on wet-paved surfaces and icy or packed snow-covered roads. Stopping times will be longer on gravel or fresh snow, although operators won't experience the dangerous lockup of wheels usually associated with conventional brakes. Drive safely because antilock brakes are only as good as the operators using them. Antilock brakes cannot compensate for driving too fast, too aggressively or failing to maintain a safe distance between vehicles. They cannot guarantee recovery from a spin or skid before braking. Also avoid extreme steering maneuvers while antilock brakes are engaged.

Your antilock braking system instrument panel tight will go on for a few seconds after starting the ignition. The tight goes on so the system can conduct the normal system test. If the tight does not go on during ignition or if the tight goes on during normal driving, this means that a problem has been detected and the antilock braking system has been shut off. Conventional braking will continue. Consult the manufacturer's service manual if this problem occurs.

Since exact antitock brake systems vary, consult the vehicle manufacturer's service and repair manuals for more details of system operation.

POWER BRAKES

Power brakes systems are designed to reduce the effort required to depress the brake pedal when stopping or holding a vehicle stationary. The booster is located between the brake pedal linkage and the master cylinder.

Most power brake systems use the difference between intake manifold vacuum and atmospheric pressure to develop the additional force required to apply the brakes. When the operator depresses the brake pedal, the power booster increases the amount of pressure applied to the piston within the master cylinder without the operator having to greatly increase brake pedal pressure.

When a vehicle is powered by a diesel engine, the absence of intake manifold vacuum requires the use of an auxiliary vacuum pump. This pump may be driven by the engine or by an electric motor.

Vacuum Boosters

On many modern vehicles, vacuum boosters are used with the hydraulic brake system to provide easier brake application. In a hydraulic brake system there are limitations as to the size of the master cylinder and wheel cylinders that can be practically employed. Furthermore, the physical strength of the operator limits the amount of force that can be applied, unless the brakes are self-energizing. These factors restrict the brake shoe to brake drum pressure obtainable. Vacuum boosters increase braking force.

A vacuum booster consists of a round enclosed housing and a diaphragm. The power brake vacuum booster uses engine vacuum (or vacuum pump action on a diesel engine) to apply the hydraulic brake system. Vacuum boosters are classified into two types (fig.7-22)& - atmospheric suspended and vacuum suspended. The descriptions of the two types are as follows:

An atmospheric suspended brake booster (fig. 7-22) has normal air pressure on both sides of the diaphragm when the brake pedal is released. As the brakes are applied, a vacuum is formed in one side of the booster. Atmospheric pressure then pushes on and moves the diaphragm.

An vacuum suspended brake booster (fig. 7-22) has vacuum on both sides of the diaphragm when the brake pedal is released. Pushing down on the brake pedal releases vacuum on one side of the booster. The difference in air pressure pushes the diaphragm for braking action.

Air has a weight of approximately 15 pounds per square inch at sea level. The weight of the air or atmospheric pressure is what is used to operate the vacuum booster.