The parking brake will not let the output shaft of the
transmission turn when the air pressure to the brake
chamber is below 40 + 5 psi (280 + 35 kPa). The brake
drum with shoes inside is on the output shaft of the
The brake chamber activates the shoes. The parking
brake is activated by a spring and released by air
pressure. Force of the spring moves the rod of the brake
chamber and the brake lever is pulled UP by the rod to
activate the brake. Air pressure works on the diaphragm
to cause compression of the spring and extension of the
rod. The rod pushes the brake lever DOWN to
RELEASE the brake.
Control Valve for Parking Brake
The control valve, which is on the dash, has control
over the supply of air that goes to the chamber of the
parking brake. With valve knob (I) pulled OUT, the line to
the brake chamber is open to the atmosphere and
causes the application of the brake. With valve knob (1)
pushed IN, air goes from the reservoir into the chamber
of the parking brake and releases the brake.
FLOW OF AIR; APPLICATION OF PARKING BRAKE
1. Knob and pin assembly. 2. Valve body. 3. Spring.
4.Orifice. 5. Outlet (to chamber of the parking brake).
6. Inlet (from air reservoir). 7. Exhaust (to
atmosphere). 8. Valve assembly.
Parking Brake Chamber
Parking brake chamber (7) releases (disengages)
and activates (engages) the emergency and parking
brake. Rod assembly (I) is connected to lever (6) which
moves the shoes in the brake. When there is no
pressure from the parking brake control valve at air
inlet(s), there is no air pressure in air chamber (3).
Springs (2) hold piston (4) all the way to the right and
keep the parking brake engaged.
PARKING BRAKE CHAMBER IN ACTIVATED
(BRAKE ON) POSITION
1. Rod assembly. 2. Springs. 3. Air chamber. 4.
Piston. 5. Air Inlet.
The brake stays activated until the air system is at the
correct pressure for operation and the parking brake
control valve is moved to the OFF position (knob is
pushed in). At this time, air pressure goes to air
chamber (3) and pushes piston (4) to the left. Rod
assembly (1) moves lever (6) which releases the parking
When air pressure decreases to approximately 40 psi
(280 kPa), the parking brake control valve closes
automatically (moves to the ON position) and stops the
flow of air to chamber (3). Springs (2) are no longer in
compression and pull rod assembly (I) into the brake
chamber. The parking brake is activated.
This same operation takes place when the parking
brake control valve is manually moved to the ON position
(pulled out). This activates the brake for parking.
LOCATION OF PARKING BRAKE CHAMBER
1. Rod assembly. 5. Air inlet. 6. Lever. 7. Parking