Here are the potential problems:
* Master Cylinder Failure: The master cylinder is the heart of the braking system. A complete failure means no brake pressure is being generated.
* Brake Booster Failure: The booster assists the driver in applying brakes. Failure means you'll need significantly more force to stop, if any at all. This is often accompanied by a very hard brake pedal.
* Brake Lines/Hoses: A leak or rupture in any part of the brake lines will result in a loss of brake fluid and pressure.
* Wheel Cylinders/Calipers: These components transfer pressure from the lines to the brake pads. Internal leaks or failures in these could prevent braking.
* Proportioning Valve: Although less likely to cause complete brake failure, a malfunctioning proportioning valve could unevenly distribute brake pressure.
* ABS System Malfunction (Less Likely): While the ABS itself shouldn't prevent all braking, a major fault *could* interfere with the system's ability to modulate pressure.
What to do immediately:
1. Do not drive the truck. Park it in a safe location, preferably on level ground and away from traffic. Engage the parking brake (if it works). If the parking brake is ineffective, chock the wheels with large, sturdy objects to prevent it from rolling.
2. Have it towed to a qualified mechanic immediately. Do not attempt to diagnose or repair this yourself unless you are a highly experienced mechanic with specialized tools and knowledge of brake systems. This is far too dangerous to risk a DIY attempt.
3. Explain the problem clearly to the tow truck driver and mechanic. Emphasize the complete lack of brakes when the engine is running.
This is a critical safety issue. Ignoring it could have devastating consequences. Get professional help right away.
