In many places, towed vehicles (trailers) which weigh more than a ton or so are required to have their own brakes. Having brakes on the trailer means that they can stop faster with less stress on the tow vehicle brakes. Trailer brakes can also be a means to control sway or other instabilities.
Most states require that trailers that weigh several thousand pounds have their own braking system. This is primarily to assure quick and safe stopping, especially in panic situations. Stopping distance is only one of several advantages.
Without trailer brakes, stopping the trailer will push the vehicle forward and try to move the tow vehicle out of its way. This can create handling problems.
Using tow vehicle brakes to stop both the tow vehicle and the trailer can put significant stresses on the brakes increasing the chance of failure and wear.
Down hill braking can create a lot of heat in brakes. Good trailer brakes help keep tow vehicle brakes from getting too hot.
The first issue is how to tell your vehicle you want the brakes applied. You usually have two methods. One is the parking brake and the other is the brake pedal. Parking brakes are not usually applied when the vehicle is in motion and travel trailers usually do not have parking brakes (this is why wheel chocks are important trailer accessories).
When you press on the brake pedal, most vehicles use that motion to put pressure on a hydraulic cylinder that then pressurizes similar cylinders on each wheel's braking system. Back in the fifties, trailers used to use hydraulic brakes and the connection was a tap to the tow vehicle brake hydraulic lines. This has been phased out for safety and convenience reasons.
Modern brake controllers separate the tow vehicle and trailer brakes. The sense and measure tow vehicle braking and then send an electrical signal back to the trailer brakes to activate them. In the seventies, these brake controllers tapped into the tow vehicle brake hydraulic lines but, again, these have been phased out due to safety reasons and due to the complexity of modern ABS systems. Other methods need to be used to sense the demand for brakes to to create an action the trailer brakes can interpret.
When you apply the tow vehicle brakes, it is going to slow down even with a trailer on behind. There are two ways to sense this motion. One is to use an accelerometer like a bob on a string and the other is to detect stress on the hitch.
U-Haul often uses surge brakes on its rental trailers. This is a mechanism on the hitch that activates the brakes as the tow vehicle pushes back on the trailer.
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