Separate Rear-wheel Parking-brake Mechanism

Separate Rear-wheel Parking-brake Mechanism

In this parking-brake shoe-expander, the hydraulic foot-brake cylinder body is bolted to the back-plate. A piston at each end actuates the shoes. A link-strut bridges the two shoes, one end connecting against one shoe web and the other end acting as the pivot point for the parking-brake lever attached to the other shoe. Two alternative lever layouts are presented in Fig. 28.44. It is perpendicular to the shoe in Fig. 28.44A and is parallel to the shoe in Fig. 28.44B. The cable is

joined to the free end of the lever. The cable pull, due to application of the hand-brake, pivots the lever. The strut, pushed by the lever one way, actuates the leading shoe and levers the trailing shoe in the opposite direction. The expanding force is shared equally between them as the link-strut floats between the two shoes.

Fig. 28.44. Separate rear-wheel hand-brake lever.

Fig. 28.45. Pressure-regulating valve.

28.8.6.

Pressure Regulating Valve

This valve (Fig. 28.45) is installed in the rear brake line. The purpose of the valve is to limit the pressure acting on the rear brakes so that the risk of rear wheel skidding is reduced. The valve uses a spring-loaded plunger enclosed in a body. Since low fluid pressure can not overcome the spring, full pressure acts initially on all brakes when the brake is applied. As soon as a predetermined pressure is reached, the valve closes and disconnects the fluid flow to rear brakes. Subsequently, any further increase of pressure is only felt by the front brakes.

28.8.7.

Brake Pressure Control Valve (Inertia Valve)

This is a pressure regulating valve. It is particularly designed to overcome the problem of the large load variation between front the rear wheels of front-wheel drive vehicles. The valve is installed in the rear brake line(s). It is an inertia sensitive pressure reducing valve. It operates when the vehicle decelerates at a predetermined rate. During this period, the valve temporarily closes the rear brake line and allows the front brake pressure to increase further. When a pre­set pressure is attained, the valve re-starts the pressure supply to the rear brakes, but at a rate much below the increase in pressure at the front brake (Fig. 28.46). The valve takes into account vehicle weight transfer and effect of attitude during braking. It is also sensitive to vehicle loading and road conditions.

Figure 28.47 illustrates the construction of a valve applicable for a normal rear brake cir­cuit. The system has independent lines using two valves mounted side by side. The valve unit uses a cylinder, fixed to the car body at a given angle. The cylinder contains a stepped piston and a steel ball. At low deceleration rates of a vehicle, fluid enters the inlet port and passes around the ball. Then it flows through the piston drilling to the rear brakes causing equal pressure in both front and rear brake lines.

Fig. 28.46. Control valve performance.

Fig. 28.47. Brake pressure control valve (Inertia valve).

If the inertia force, produced by the rate at which the vehicle slows down, rolls the ball up along the sloping cylinder, then the ball stops the fluid supply to the rear brakes. During this period, the difference in piston area maintains the outlet pressure constant while the inlet pressure is being increased. At a particular point, depending upon the piston areas, an increase

in inlet pressure moves the piston providing a propor­tional pressure to the rear brakes. Pressure in the two lines at this stage is governed by the relation ; Inlet pressure x Small area = Outlet pressure x Large area.

28.8.8.

Pressure Differential Warning Actuator

This warning device illuminates a brake failure warning lamp when the pressure difference in the two brake lines differs by more than a specified amount. When failure of one brake line occurs the pistons (Fig. 28.48) moves and operates the electrical switch. The switch remains closed until the pistons are reset.

28.8.9.

Load-apportioning Valve

This valve, within specified limits, provides hydraulic pressure to the rear brakes in proportion to the load carried by the rear wheels. Therefore this reduces the risk of rear wheel skidding when the rear of the vehicle is lightly loaded. Also, this arrangement ensures

Fig. 28.48. Pressure differential warning actuator

good braking when the rear wheels are over loaded. One load-apportioning valve is sufficient for a single hydraulic line layout, but a separate valve is required in each line when a diagonal line system is incorporated. The valve housing is mounted on to a rigid part of the vehicle body. A spring working either in tension or compression senses the load on the rear wheels. This spring connects the valve-operating lever to a part of the suspension system that moves in proportion to the vehicle load.

The construction of the valve is shown in Fig. 28.49. The lever acts directly on a piston, which uses a ball valve. In the released position of the brake, the piston lies at its bottom position and the ball valve is held open by a push-rod attached to the valve body. This allows the fluid to pass freely between the inlet and outlet ports. As the hydraulic pressure is applied to the valve the upward movement of the piston takes place. This is accomplished by providing a larger area exposed to the fluid on the upper part of the piston than the area on the bottom part. The amount of hydraulic pressure, required to raise the piston and close the ball valve is governed by the force exerted by the external spring on the piston.

If the load on the rear wheels is light, only a small force is exerted by the spring on the piston. Consequently, a relatively low pressure is required to move the piston upwards to close the valve. If the pressure at this closure point is exceeded, full pressure cannot be applied to the rear brake. Therefore any further increase in pedal force causes the piston to control the valve to maintain a lower pressure that is also proportional to the pressure applied to the front brake.

As the load on the rear wheels increases, the suspension deflects and the force on the external spring increases. Therefore, to face this extra force exerted by the spring on the piston, a higher fluid pressure is created before the piston is able to rise. As a result full pressure on the rear brakes is maintained until a much higher pedal force is applied.

Fig. 28.49. Load-apportioning valve (Bedix).

The type of valve presented in Fig. 28.49 uses an adjusting screw between the lever and piston. This controls the point at which the valve comes into operation, determining the front/rear braking ratio for a given rear wheel load.

Besides any leakage of the seals, fracture of the external spring may be the possible faults in this arrangement. The breakage of the spring results in considerable reduction in the pressure supplied to the rear brakes through the valve. The complete valve unit is usually replaced, when it is found defective.

28.8.10.