Figure 4-151. Power-Boost Master Cylinder (Brake Released)
port. Whatever amount of pressure is present in the wheel brake assembly at any given moment is also present in the
hollow of the spool, having entered the spool through a cross-drilled hole. This pressure within the spool tends to move it
toward the right. This is in opposition to the force exerted by the spool pressure spring and reblocks the pressure port
The amount of fluid pressure that has to be present at any given time in the wheel brake assembly and in the spool to
cause reblocking of the pressure port depends on the amount of force in the spool pressure spring at that time. Since the
amount of force exerted by the spool pressure spring is in proportion to the distance the brake pedal is depressed, the
degree of pressure with which the brake is applied depends on how far the pedal is depressed.
Pressure maintenance. Once a given degree of braking effort has been obtained by
depressing the pedal a given distance, it will remain unchanged as long as the pedal is not moved. This is because the
movement of the spool to the right in blocking the pressure port leaves the brake line isolated from both the pressure and
the return lines. Thus, no fluid can enter or leave the wheel brake assembly and change the amount of pressure therein
unless the position of the spool is changed by moving the pedal. When the brakes are in this static condition, the
operator may experience the illusion that he is still applying the brakes. The reason for this is that the fluid pressure
within the spool that tends to move the spool toward the right is transmitted back to the brake pedal through the spool
pressure spring and the plunger. This results in a push of the pedal against the operator's foot, giving him the feeling that
he is applying the brakes.
Brake release. As shown in figure 4-153, releasing the brake causes the plunger spring
to move the plunger toward the right. This releases pressure on the spool pressure spring and permits the spool return
spring to move the spool toward the right. Under these conditions the pressure port is blocked and the brake line and the
return line are interconnected. The fluid that was forced into the wheel brake assembly when the brake was applied now
returns to the reservoir.
Wheel Brake Assemblies. The wheel brake assembly is that portion of a wheel brake system that
receives pressure from a master cylinder or a power brake control valve and converts the pressure into a retarding force
that stops wheel rotation.
Disc brakes. A typical disc assembly is shown in figure 4-154. In a disc assembly, one or two
steel discs are generally used, depending on the type