Hydroboost brake systems use pressure from the power steering system instead of vacuum as a source of power assist. Manufacturers use hydraulic fluid pressure from the power steering system instead of vacuum because not all engines produce enough to provide assistance. This is true for diesel, turbocharged, and supercharged engines. A loss of power steering pressure will have a significant effect on the Hydroboost brake assist booster.
Diagnosing a Hydroboost brake booster is similar to a vacuum-operated booster, except an inspection of the power steering system is in order. Check the level of the power steering fluid reservoir. Inspect the power steering belt and pump and the lines and hoses for leaks.
With the engine off, pump the pedal several times to discharge the accumulator. Then, hold the brake pedal to the floor and maintain pressure while starting the engine. The pedal should move slightly toward the floor and back toward your foot.