Several things could cause this:
* Master Cylinder Failure: This is a common culprit. The master cylinder is the heart of your braking system, and if it fails, you lose pressure. Internal seals can wear out or it can crack.
* Brake Booster Failure: The booster assists you in applying the brakes. A failure means you'll have to exert significantly more force on the pedal, and if it's completely failed, you'll go straight to the floor. A vacuum leak in the booster line is also a possibility.
* Brake Lines/Hoses Leak or Rupture: A leak in any part of your brake lines (from the master cylinder to the calipers) will result in a loss of pressure. This could be due to corrosion, damage, or a loose connection.
* Wheel Cylinder Failure (Rear Brakes): A failure in one or more of the wheel cylinders (which push the brake shoes in the rear drums) can cause a loss of pressure, although this often manifests more gradually.
* Caliper Piston Seizure (Front Brakes): A seized caliper piston can prevent the brake pad from retracting, leading to dragging and reduced overall brake pressure.
* ABS System Malfunction (Less Likely): While less common for a complete loss of braking, an ABS module failure *could* contribute to a significantly reduced braking capacity in certain circumstances. It's less likely to be the sole cause of a complete loss of pressure.
* Low Brake Fluid: While unlikely to cause an *instantaneous* loss of pressure, critically low brake fluid indicates a leak somewhere in the system. Check your brake fluid reservoir - if it's empty or extremely low, you have a serious leak somewhere. Do not simply add fluid without determining the source of the leak first.
What you should do:
1. Do not drive the truck. This is crucial for your safety and the safety of others.
2. Have it towed to a qualified mechanic. Attempting to diagnose and repair this yourself is extremely dangerous.
3. Before towing, check the brake fluid reservoir. If it's empty or very low, that strongly suggests a leak. Note the level and any signs of fluid leaking near the brake lines or components.
A mechanic will be able to diagnose the precise cause of the brake failure and perform the necessary repairs. This is not something to take lightly. Your safety depends on it.
