NOCTI Automotive Practice Test

All four wheels locking up points to a problem that affects hydraulic pressure to every wheel. The master cylinder is the component that converts pedal effort into hydraulic pressure and distributes it to all four wheel circuits. If the master cylinder is failing or sticking, it can keep the pressure high across the entire braking system, causing simultaneous lock-up of all four wheels when you brake. The ABS module’s role is to modulate pressure to prevent lock-up; a problem there could lead to unexpected braking behavior, but the symptom described is most consistent with a master-cylinder fault. Brake lines that leak would cause a loss of pressure and reduced braking power, not locking. The proportioning valve mainly adjusts rear versus front pressure; a fault there tends to cause imbalanced braking rather than lock-up of all wheels.