Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairHard
A technician is diagnosing a heavy-duty truck equipped with an air-over-hydraulic brake system. The driver reports a hard brake pedal and slow brake release. An inspection reveals that the air compressor is building sufficient pressure, and there are no external air leaks. Which of the following components is MOST likely causing these symptoms?
- AA faulty air-over-hydraulic slave cylinder.
- BA restricted air dryer.
- CA malfunctioning treadle valve (foot valve).
- DA collapsed hydraulic brake hose.
Show answer & explanationAnswer & explanation
Correct answer: C. A malfunctioning treadle valve (foot valve).
In an air-over-hydraulic system, the treadle valve (foot valve) controls the air pressure sent to the air-over-hydraulic slave cylinders. If this valve is malfunctioning, it can restrict air flow, leading to a hard pedal (insufficient air pressure to the slave cylinders) and slow release (valve not fully exhausting air).
Why the other options are wrong
- A. A faulty slave cylinder would typically cause a lack of braking at that wheel, not a hard pedal for the entire system or slow release.
- B. A restricted air dryer would cause a general reduction in air pressure or moisture issues, but if the compressor is building sufficient pressure, it's less likely the direct cause of these specific symptoms.
- D. A collapsed hydraulic brake hose would cause a lack of braking at a specific wheel and potentially a pull, not a hard pedal for the entire system or slow release, especially if air pressure is the primary assist.
Air-Over-Hydraulic Treadle Valve
The foot-operated valve in an air-over-hydraulic brake system that controls the flow of compressed air to the air-over-hydraulic slave cylinders.
- Directly controls brake application and release via air pressure.
- Malfunction can cause both hard pedal and slow release.
- Crucial for proportionate braking control.
Memory trick: Treadle valve is the 'air faucet' for the brakes; if it's stuck, flow is bad.