There is very little pedal travel with air brakes. You need to picture that unlike hydraulic brakes where you are providing the hydraulic pressure by stepping on the brakes, with air brakes you are merely opening a valve to allow air to go to the brake chambers. When you let off the brakes, air escapes through a quick release valve. The trailer air is controlled by what is called a tractor protection valve. It has 2 outlet ports. The emergency or supply port which has constant air pressure going to the trailer to supply the trailer with air and to keep the trailer brakes released. The service port is the port that supplies air to the trailer brakes when you apply the brakes. Chevrolet was still just a rigged up straight truck and was really not engineered for tractor equipment as were the heavy duty trucks in 1966. Many did not have a tractor protection valve and merely had 2 ball valves to cut off the trailer air when bob tailing. If you have a tractor protection valve, there should be a toggle switch on the dash somewhere. Also you may or may not have a trolly valve on the steering column which you can use for extra braking power to the trailer. In my opinion, the swinging brake pedal Chev used is difficult to use because with floor mounted brake pedal, you rest your heel on the floor and learn to apply braking power with your ankle. With swinging pedals, you have to regulate the pressure with your whole leg and it is easy to apply too much air in a panic situation and wind up in a skid. I guess my main point is that in 66 Chev was not engineered as a truck tractor from the ground up like the heavy trucks. Many were retrofitted with tractor equipment, and air brake principles were poorly understood by most people who installed the tractor equipment and many systems were poorly designed. Probably the best thing to do would be to find some old timer who worked on and understood the systems of that period to find out what you have. If it were mine, I would not worry about staying original, but rather retrofit the brake system with more modern components for safety.
