Low brake fluid, drums/rotors in need of replacement, pads in need of replacement, air in the brake lines, worn master cylinder, brakes out of adjustment... all possible reasons.
replace brake booster
If your brake peddle goes to the floor, you have lost brake fluid from your master cylinder in the engine compartment. Check your rubber brake hoses at each of your wheels, look for any signs of leaking brake fluid at your wheels, connections,lines, and master cylinder.
soft peddle feel maybe from a damaged brake master cylinder. this pressurizes the peddle, and if there is a leak in the cylinder it might feel soft or flop all the way to the floor.
Press and release the foot peddle that is located on the floor
I had the same problem, on top of the brake peddle there is a switch that you have to replace, it is pretty clear with a flashlight, It knows when the peddle goes down, in my car it was white.
A leak in your brake line, or very low brake fluid, both of which are extremely dangerous and should be fixed at once.
a leak or a bad master cylinder
you might have fluid leaking before that.. lots of press no volume
If a truck has a clutch brake it would only be used when coming to a complete stop. By pushing the clutch peddle completely to the floor you would apply the clutch brake. This stops the clutch disc from spinning making it easier to shift into 1st or reverse gear. Normal shifting while driving you would not need to or want to push the clutch peddle to the floor to activate the clutch brake.
Remove and replace the fuses one at a time whilst pressing the brake peddle until the lights go out.
Your brake light switch is out of adjustment. Don't know what vehicle you have but most brake light switches are located on the brake peddle lever just above the brake peddle pad.
Brake pads could be worn or lack of fluid in brake funnel