Figure12-28.-Power brake control valve (sliding spool type).
system from the normal brake system. When brake
the brakes are applied. The seal is held in place by a
actuating pressure enters the shuttle valve, the shuttle is
automatically moved to the opposite end of the valve.
This action closes off the inoperative brake system
actuating line. Fluid returning from the brakes travels
back into the system to which the shuttle was last open.
Power Brake Control Valve
(Pressure Ball Check Type)
A power brake control valve of the pressure ball
check type is shown in figure 12-27. The valve is
designed to release and regulate main system pressure
to the brakes and to relieve thermal expansion when the
brakes are not being used. The main parts of the valve
are the housing, piston assembly, and tuning fork.
The housing contains three chambers and three
ports. They are the pressure inlet, brake, and return
ports.
The piston assembly is made up of a piston head,
piston shaft, pilot pin, and cross pin. The piston head
separates the brake and return chambers. A cup seal
prevents fluid from escaping to the return chamber when
retainer and piston return spring. The piston head has a
hole drilled through its center for the flow of fluid to the
return port. This hole is opened and closed by the pilot
pin. The pilot pin also opens the pressure port. The
flange of the pilot pin and the hole in the piston head are
lapped together. The piston shaft connects the piston
head with the tuning fork. The shaft is slotted, and the
cross pin prevents it from turning.
The tuning fork connects the brake pedal linkage
with the control valve. It swivels on the housing and
limits the maximum pressure directed to the brake. The
upper arm of the tuning fork is a bar spring that bends
from the point of the fulcrum when hydraulic pressure
overcomes toe force.
Power Brake Control Valve
(Sliding Spool Type)
A sliding spool-type power brake control valve is
shown in figure 12-28. This valve consists basically of
a sleeve and a spool installed in a housing. The spool
moves inside the sleeve, opening or closing either the
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