Parking Brake Control Valve and Trailer Air Supply Valve
Notice: Do not step on the service brake pedal while the parking brakes are applied. To do so can cause damage to the brake mechanisms.
The yellow diamond-shaped knob operates the parking brake valve. Pulling the knob applies both the tractor and the trailer spring parking brakes. See Fig.. Pushing the knob in releases the tractor spring parking brakes. Before the spring parking brakes can be released, the air pressure in either air brake system must be at least 65 psi (447 kPa).
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Trailer Air Supply Valve Control
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Parking Brake Control Valve Control
Fig. 1, Dash Brake Valve Controls
The red octagonal-shaped knob operates the trailer air supply valve. See Fig.. After the vehicle and its air hoses are connected to a trailer and the pressure in the air system is at least 65 psi (447 kPa), the trailer air supply valve knob must be pushed in (and should stay in) to charge the trailer air supply system and release the trailer spring parking brakes. Before disconnecting a trailer or when operating a vehicle without a trailer, the trailer air supply valve knob must be pulled out.
See General Information, Brake System, for complete brake operating instructions.