A heavy hauler equipped with an air-over-hydraulic braking system exhibits normal air pressure readings, but the hydraulic pressure at the wheel cylinders is significantly lower than specified. The master cylinder is full, and no external leaks are detected. What is the most probable cause?

Question: A heavy hauler equipped with an air-over-hydraulic braking system exhibits normal air pressure readings, but the hydraulic pressure at the wheel cylinders is significantly lower than specified. The master cylinder is full, and no external leaks are detected. What is the most probable cause?

Answer options: ✅ A faulty air-over-hydraulic intensifier (hydrovac) unit.

• Improperly adjusted pushrod between the air chamber and master cylinder.
• A seized hydraulic caliper piston.
• Contaminated air in the air supply reservoir.

Correct answer: A faulty air-over-hydraulic intensifier (hydrovac) unit.

Explanation: The air-over-hydraulic intensifier, also known as a hydrovac or air-hydraulic booster, uses air pressure to multiply force and create high hydraulic pressure. If this unit is faulty, it will fail to adequately convert the air pressure into sufficient hydraulic pressure, leading to low pressure at the wheel cylinders despite normal air supply. An improperly adjusted pushrod could cause various issues, but not specifically low hydraulic pressure if there's no air issue. A seized caliper piston would generally lead to locked brakes or no braking at that wheel, not systemic low pressure. Contaminated air would primarily affect air components.