Releasing - No. 1 Circuit Portion
With the brake treadle released, mechanical force is removed from the spring seat, rubber graduating spring, and No. 1
piston. Air pressure and spring load moves the No. 1 piston, opening the No. 1 exhaust valve allowing air pressure in the
No. 1 delivery line to exhaust out the exhaust port.
Releasing No. 2 Circuit Portion
With the treadle released, air is exhausted from the No. 1 circuit side of the relay piston. Air pressure and spring load
move the relay piston, opening the No. 2 exhaust valve allowing air pressure in the No. 2 delivery line to exhaust out the
Low Pressure Indicating Switch
The low pressure indicating switch is designed to provide a warning to the operator that one or both air systems are at a
low pressure. The switch is electrically connected to a buzzer and a LED indicator light. The switch has a die cast body
with a nylon cover, and employs a spring loaded O-ring diaphragm and piston. A gasket is used between the cover and
body. The switch is provided with electrical contacts and a single terminal from connection to the electrical system. The
contacts remain closed by spring force until the air pressure below the diaphragm is great enough to overcome the spring
force. This pressure setting is approximately 75 psi (517 kPa/5.17 bar) and should be marked on a label affixed to the
When air pressure at the supply port and under the diaphragm is above 75 psi (517 kPa/5.17 bar), the electrical contacts
remain open because the force exerted by air pressure underneath the diaphragm overcomes the force exerted by the
spring above the diaphragm.
When air pressure below the diaphragm drops below 75 psi (517 kPa/5.17 bar), the spring exerts a force which is greater
than the force exerted by the air pressure below the diaphragm. This causes the diaphragm and piston to move and allow
the electrical contacts to close. This completes or closes the electrical circuit to the warning buzzer and the indicator light.
Stop Light Switch
A stop light switch is installed in the No. 1 circuit of the dual brake valve. The switch senses the pressure in the brake air
lines. Electrically, the switch is connected to illuminate the service stop lights on the rear of the machine with the MS51113
switch in the SERVICE Mode, when the brake pedal is pressed. With the MS51113 switch in the BLACKOUT Mode, the
stop light illuminates the blackout stop lights when the brake pedal is pressed. The switch is an electro-pneumatic switch
and is not a serviceable item. It must be replaced as a complete assembly.