Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairHard
A vehicle equipped with a hydro-boost brake system exhibits a hard brake pedal and a noticeable increase in steering effort when the brake pedal is depressed. The power steering fluid level is correct, and the power steering pump appears to be functioning normally at idle. Which component is MOST likely at fault?
- AThe hydro-boost unit's power piston.
- BThe master cylinder's primary seal.
- CThe power steering pump's flow control valve.
- DThe brake pedal position sensor.
Show answer & explanationAnswer & explanation
Correct answer: C. The power steering pump's flow control valve.
The power steering pump's flow control valve regulates the hydraulic pressure delivered to both the steering gear and the hydro-boost unit. If this valve malfunctions, it can restrict flow and pressure, leading to reduced assist for both braking and steering, particularly under load (like pressing the brake).
Why the other options are wrong
- A. A faulty power piston in the hydro-boost unit would primarily affect braking assist, not steering effort simultaneously.
- B. A master cylinder primary seal leak would cause a spongy pedal or a pedal that sinks, not a hard pedal combined with increased steering effort.
- D. A brake pedal position sensor typically affects cruise control, ABS, or automatic transmission shift logic, not hydraulic assist for braking or steering.
Hydro-Boost Flow Control Valve
A valve within the power steering pump that regulates hydraulic fluid flow and pressure, supplying both the power steering gear and the hydro-boost brake unit.
- Crucial for proper operation of both steering and hydro-boost.
- Malfunction can cause reduced assist to both systems simultaneously.
- Often a spring-loaded plunger that diverts flow.
Memory trick: Two systems, one pump, one shared valve. If one fails, both suffer.