Final answer:
The most likely cause for all the brakes dragging on a truck with hydraulic brakes is a leaking secondary cup in the master cylinder. If the secondary cup leaks, it can cause the brakes to remain engaged and create excessive friction.
Step-by-step explanation:
The most likely cause for all the brakes dragging on a truck with hydraulic brakes is (C) a leaking secondary cup in the master cylinder. When the secondary cup in the master cylinder leaks, it can cause the brakes to remain engaged and create excessive friction, resulting in the brakes dragging.
If there is air in the hydraulic system (A), it can result in a spongy brake pedal feel and reduced braking performance, but it wouldn't cause all the brakes to drag. Too high vacuum supply to the power brake booster (B) would not cause the brakes to drag either. No brake pedal free travel (D) would not cause all the brakes to drag, but it can cause excessive brake wear or premature brake engagement.