THEORY OF OPERATION - CONTINUED
BRAKE SYSTEM - CONTINUED
Modulation of pressurized oil in service brake accumulators is controlled by service brake control valve. Position
of either brake pedal causes a specific pressure at service brakes. As position of pedal changes, pressure at service
brakes also changes.
When operator pushes brake pedal, a roller on brake pedal assembly forces piston downward. Piston applies a
force on springs. Force of springs moves retainer and ball. Movement of retainer and ball causes upper valve spool
to move away from seat in spacer.
Movement of upper valve spool causes movement of lower valve spool and compression of spring. When valve
upper and lower valve spools move, oil flow from outlet, to service brake passages, to hydraulic oil tank is
This allows pressurized oil supply from accumulator ports to flow through upper and lower valve spool passages
and outlets to service brakes. Oil then flows through outlet to service brake passages to apply service brakes, while
at same time, pressurized oil flows into cavities through outlets to service brakes. Oil pressure at service brakes has
same pressure as oil in cavities.
Oil pressure in cavity and force of spring act to balance lower valve spool against force of pressure in cavity. In
same manner, pressure in cavity creates a force against bottom of upper valve spool. Upper valve spool is balanced
by force of springs at top of upper valve spool.
Force of springs is balanced by force applied to pedal. Upward movement of valve upper and lower valve spools
causes supply from accumulator ports to be restricted. Level of pressure in independent valve sections creates a
force of feedback. This allows operator to modulate pressure to service brakes.
Valve upper and lower valve spools balance between supply from accumulator ports and ports for service brakes to
maintain brake pressure until position of pedal changes. Change in pedal position means that more or less brake
pressure is necessary.
If piston moves downward to apply more compression on springs, valve upper and lower valve spools move to
allow more pressure oil to flow to service brakes outlets. This results in a higher oil pressure in cavities and service