installed in the pilot's and copilot's brake actuating
brakes are not being used. The main parts of the valve
lines. The check valves allow the flow of fluid in one
are the housing, piston assembly, and tuning fork.
direction only. The orifice check valves allow
The housing contains three chambers and three
unrestricted flow of fluid in one direction, from the
ports. They are the pressure inlet, brake, and return
pilot's brake valve; flow in the opposite direction is
ports.
restricted by an orifice in the poppet. The purpose of
The piston assembly is made up of a piston head,
the orifice check valves is to help prevent chatter.
piston shaft, pilot pin, and cross pin. The piston head
The next unit in the brake actuating lines is the
separates the brake and return chambers. A cup seal
pressure relief valve. In this particular system, the
prevents fluid from escaping to the return chamber
pressure relief valve is preset to open at 825 psi to
when the brakes are applied. The seal is held in place
discharge fluid into the return line. The valve closes at
by a retainer and piston return spring. The piston head
760 psi minimum.
has a hole drilled through its center for the flow of fluid
Each brake actuating line incorporates a shuttle
to the return port. This hole is opened and closed by the
valve for the purpose of isolating the emergency brake
pilot pin. The pilot pin also opens the pressure port.
system from the normal brake system. When brake
The flange of the pilot pin and the hole in the piston
actuating pressure enters the shuttle valve, the shuttle
head are lapped together. The piston shaft connects the
is automatically moved to the opposite end of the
piston head with the tuning fork. The shaft is slotted,
valve. This action closes off the inoperative brake
and the cross pin prevents it from turning.
system actuating line. Fluid returning from the brakes
The tuning fork connects the brake pedal linkage
travels back into the system to which the shuttle was
with the control valve. It swivels on the housing and
last open.
limits the maximum pressure directed to the brake.
The upper arm of the tuning fork is a bar spring that
Power Brake Control Valve (Pressure Ball
bends from the point of the fulcrum when hydraulic
Check Type)
pressure overcomes toe force.
A power brake control valve of the pressure ball
Power Brake Control Valve (Sliding Spool
check type is shown in figure 14-6. The valve is
Type)
designed to release and regulate main system pressure
to the brakes and to relieve thermal expansion when the
A sliding spool-type power brake control valve is
shown in figure 14-7. Basically, this valve consists of a
sleeve and a spool installed in a housing. The spool
moves inside the sleeve, opening or closing either the
pressure or return port of the brake line. Two springs
are provided. The large spring, referred to in the
illustration as the plunger spring, provides "feel" to the
brake pedal. The small spring returns the spool to the
OFF position.
When the plunger is depressed, the large spring
moves the spool, which closes off the return port and
opens the pressure port to the brake line. When the
pressure enters the valve, fluid flows to the opposite
end of the spool through a hole. The pressure pushes
the spool back far enough toward the large spring to
close the pressure port, but not open the return port.
The valve is then in the static condition. This
movement partially compresses the large spring,
giving "feel" to the brake pedal. When the brake pedal
is released, the small spring moves the spool back,
opening the return port. This action allows fluid
pressure in the brake line to flow out through the return
Figure 14-6.--Power brake control valve (pressure ball check
port.
type).
14-5
