1. How they work:

When you push on the brake pedal, hydraulic pressure is generated in the master cylinder that is transmitted to the actuators -the wheel cylinders and caliper pistons- through the brake lines, thus applying the brakes.

The operation of the braking system depends on the integrity of the hydraulic system, the master cylinder must be capable of generating several hundred pounds per square inch of pressure and the rest of the system must maintain the pressure without leaking.

Most automotive systems in use today utilize front disk brakes and rear drum brakes, but 4 wheel disk systems are also fairly common. With disk brakes, the rotor spins with the wheel and the pads move out and rub the rotor when the brakes are applied. (See diagram) Most disk brakes use floating calipers, the caliper slides in and out as the brakes are applied and released. The piston moves the inside pad out and pushes the outside pad into the rotor by sliding the caliper back toward the rotor. With a drum brake assembly, the drum, shaped like a pie plate, rotates with the wheel and the fixed shoes move out to engage the drum when the brakes are applied.

Most systems use a booster that uses engine vacuum to increase the force applied to the brake pedal. (Diagram) Disk brakes require higher application pressure than do drums because they are not self actuating. When drum brakes are applies the lead shoe helps bring the trailing shoe into the drum. (Servo Action)

2. ABS

Antilock brake systems use a computer to monitor and control the braking system. (Diagram) Essentially the computer monitors the rotational speeds of the wheels and then releases the brakes when the wheels are about to lock up. Wheel lock up is undesirable for two reasons:

1. The braking system including the tires is most effective, i.e., produces the most retardation, before the wheels lock up and start to slide. So it is beneficial to keep the wheels rotating at a rate of about 5%-10% of the free rolling velocity for the vehicle's speed. This improvement in braking efficiency, while present, is actually a secondary benefit of ABS however.

2. Locked wheels cannot provide for directional or rotational control of the vehicle. If the rear wheels lock, the vehicle will tend to spin out toward the end of the braking interval. If the front brakes lock up, the vehicle cannot be steered. It is probably this second aspect of the ABS effect on vehicle control that is most highly publicized, but the actual benefit of these systems for accident prevention or injury reduction is yet to be proven..

The vehicles most in need of ABS are light trucks. The braking systems of these vehicles are frequently biased toward the rear to accommodate the payload. But when the vehicle is lightly loaded there can be too much rear bias resulting in early wheel lockup and loss of control. Pickups in particular are vulnerable to this problem. All pickups should at least be equipped with a load sensing valve to adjust rear brake pressure (Diagram) but most are not.

3. Brake Problems:

All modern brake systems use a split hydraulic system. The system is split in the master cylinder (Diagram) and this prevents the complete loss of brakes if a leak develops on one side of the system. Complete loss of brakes is therefore very unlikely except for unusual problems with the master cylinder. However, if the front brakes are lost, the vehicle will be down to 30% to 40% of normal braking capacity and it may feel to the operator as though there is a complete loss of brakes.

Many brake system failures are caused by the introduction of water into the braking system. Water causes rust and corrosion in the metal parts of the system and makes it difficult for the seals to operate correctly and produce the high pressures needed for braking.

Grabbing brakes are most often caused by contaminated linings. Grease or oil can cause the linings to swell at spots and on drum brake systems these swollen spots contact the drum too quickly and can cause the brakes to grab.

Noisy brakes may or may not indicate problems. Sometimes a brake squeal is an indication that the linings or pads are worn badly enough to allow metal to metal contact. Disk brakes can squeal if they are improperly installed.

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