Hydroboost brake systems use pressure from the power steering system instead of vacuum as a source of power assist. Manufacturer's use these instead of vacuum because not all engines produce enough vacuum to provide assist. This is true for diesel, turbocharged and supercharged engines. A loss of power steering pressure will have a great 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 power steering belt and inspect 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 then move back toward your foot.