To reduce application pressure to the steering axle by 50 % compared to the drive axle brakes, when required by reduced road conditions (rain, snow or ice).
The system consists of a dash switch with 2 positions:
Dry Road: 100% front wheel braking; in other words, all brakes apply equally.
Slippery Road: reduces braking force to the front brakes by (50%) when in the slippery position.
The brake pedal applies the service brakes - truck, or truck and trailer brakes.
In other words, the brake pedal brings the vehicle to a stop when it it depressed.
The foot valve operates the secondary and primary sub systems simultaneously.
Failure in one part of the dual air system does not affect the operation of the other, owing to the division of the braking system into two independent subsystems.
The brake pedal valve is also a self-balancing valve.
If there is a 10psi (69kPa) brake application made, and a slight leak exist in one part of the system that causes a reduction in the 10psi brake application, the foot valve will reopen and top up the air pressure that part of the system.
In other words, even if there is a leak, the foot valve and other valves will compensate for this by maintaining a 10psi brake application pressure.
The brake pedal is a valve. It is called either the foot valve or the treadle valve.
As well, there is one located at the trailer air tank.
The one-way check valves at the entrance of the primary and secondary tanks are primarily responsible for dividing the system into two independent subsystems.
If there is a leak in either subsystem, the one-way check valves prevent air from bleeding out from the subsystem that still maintains air pressure.
To keep the parking brakes off, the two-way check valve is designed to pull air from the system with the higher pressure; therefore, the spring parking brakes will not apply automatically in the event that one of the subsystems fails.
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To reduce application pressure to the steering axle brakes during most braking situations. Reduced pressure to the steer axle brakes occurs for most braking applications. Only during hard braking is there equivalent pressure delivered to both the front and…
MUST KNOW The brake pedal applies the service brakes - truck, or truck and trailer brakes. In other words, the brake pedal brings the vehicle to a stop when it it depressed. SHOULD KNOW The foot valve operates the secondary and primary sub systems…
OBSOLETE To reduce application pressure to the steering axle by 50 % compared to the drive axle brakes, when required by reduced road conditions (rain, snow or ice). The system consists of a dash switch with 2 positions: Dry Road: 100% front wheel braking; in…
A cattle chute with one-way gates allows the cows to only move in one direction – forward. A one-way check valve in an air brake system works exactly the same – it allows air to only move in one direction – forward through the system. The one-way check valves…
OBSOLETE Fills the trailer air tanks (reservoir) and applies and releases the trailer service brakes. The relay emergency valve also applies the trailer emergency brakes in the event of a trailer breakaway or normal disconnect. Knowledge Test Question: In the…
OBSOLETE Used on some vehicles with dual air systems. This device provides a means of using the force of the spring brakes to assist in stopping a vehicle in an event of a failure in the rear (primary) air system.