Trailer brake systems are operated by a brake controller such as the Tekonsha Prodigy and have nothing to do with the tow vehicle's own brake system except to be triggered by use of the brake pedal.
Brake controllers have various methods of determining how much trailer braking to apply. The most common ones are a simple timed algorithm (the longer the brake pedal is down, the longer/greater the trailer brakes are applied) or a sensing system based on acellerometers, which determine how fast the tow vehicle is decellerating and applying trailer braking proprtionally. The Tekonsha Prodigy is an excellent example of the proportional type.
To be sure I understand, a brake controller such as the Tekonsha Prodigy is installed and it's action determined by brake pedal travel, is that right?
My experience with a brake controller goes back to the mid 70's when we toed a TT and the brake controller was connected to the hydraulic brake line. There was a manual contol handle in the car for manual overide.
The Prodigy has a sensing lead that goes to the tow vehicle's brake lights to determine when the vehicle is braking. The controller also has an intertial sensor that senses how fast the vehicle is decelerating and sends a proportionate amount of current to the trailer brakes.
The MasterBrake controller used a zero displacement transducer that measured the vehicle's hydraulic pressure to determine how much power to send to the trailer brakes. They also had an air brake version, but I don't know if the company is still in business.
Hayes still makes a "traditional" controller that uses air pressure to send electric current to the trailer brakes. It works just like the old hydraulic versions and will give better braking control than a time or intertia based controller. Take a look at http://www.brakecontroller.com and scroll down towards the bottom of the page.