In order for a tractor-trailer truck to stop in a straight line, a special relay device termed a brake actuator is needed as part of the rear brake assembly. It fires a burst of air into the rear brake assembly at the same moment the driver touches the brake pedal. Its action is typified by a loud, short hiss when the brakes are applied. It recharges as soon as the brakes are released and the truck begins to roll.
The brake actuator is a small mechanically operated device that is a key part of the rear air brake assembly of a trailer. Its sole purpose is to ensure that the brakes are applied at the same instance on both the tractor and trailer. If there is a momentary lag between the applications of brakes, then it is possible a jackknife could occur. This scenario would occur if the trailer’s rear brakes were applied before the tractor stopped. This, in turn, would force the rear eight wheels of the trailer to grab first and a skid could develop with catastrophic results.
This is analogous to the brake equalization systems that are used in front-wheel drive cars. In a front-wheel drive car, 69% of the weight is over the drive wheels and the front brakes, when applied, act almost instantly. The rear wheels and brakes, if there were no brake equalization system, would not act as quickly and, again, a skid could develop from the imbalance. In order to equalize brake action, your car is equipped with a system that equalizes front and rear braking action and prevents skids.
In a tractor-trailer truck, the brake actuator system is enabled after the air, hydraulic and electrical lines from the front tractor rig are attached to the correct connectors on the trailer rig. On activation, the front brakes act almost instantly, while there would be a bit of lag time to the rear brake assembly. This would throw the entire truck out of balance, possibly causing the rear wheels to lock up and start the trailer to skid to one side or the other. The brake actuator eliminates this lag time.
To equalize the braking force applied, a small electrical relay is tripped that “tells” the brake actuator to put a small blast of air into the rear braking assembly, equalizing all braking forces. This relayed signal insures that the entire 60-foot tractor-trailer remains stable as the braking force is applied at the same instant.
In tractor-trailers equipped with brake actuator systems, there is an additional gauge on the instrument panel that is important to the driver, the brake application gauge. This gauge indicates to the driver how much air pressure is left in the brake actuator system. If the system is working correctly, the gauge should remain in the upper or middle area of the dial.
However, if there is a problem – for example, a slow leak – then the gauge will remain on the low side of the dial and will not move when the system is recharging. Recharging occurs when the tractor-trailer starts off from a complete stop and the brake actuator gauge will always remain in the middle to upper end of the dial, if the system is working correctly.