Power Brake Booster Function Test (Vacuum, Hydro-Boost & Hydro-Max): Notes

WARNING: This page does not describe the selected car, but rather 14 other vehicles, including the 2004 Ford RV Cutaway, 2004 Ford Econoline, 2004 Ford E450 Super Duty, 2004 Ford Cutaway, and 2003 Ford RV Cutaway. However, it is still accessible from the selected car via links, so may be relevant.

With engine stopped, remove vacuum in system by pumping brake pedal several times.
Hold the brake pedal in the applied position. Start the engine.
The brake pedal should drop slightly after the engine starts.
On vacuum brake booster equipped vehicles, if the brake pedal does not drop after the engine is started, check for vacuum at the brake booster. With the engine OFF, disconnect the vacuum hose and check valve at the brake booster. Vacuum should be heard escaping from the brake booster.
If vacuum is not available at the brake booster, check for vacuum at the source and check the vacuum hose for leaks. On vehicles without a vacuum pump, check for vacuum at the engine. On vacuum pump equipped vehicles, inspect the accessory drive and accessory drive belt.
If vacuum is available at the brake booster and no power assist is felt, install a new brake booster and retest the system.
On Hydro-Boost equipped vehicles, if the brake pedal does not drop after the engine is started, check the power steering pump pressure and flow.
If power steering pump pressure is available to the Hydro-Boost and no power assist is felt, install a new Hydro-Boost brake booster and retest the system.
On a Hydro-Max booster system, if power is available at the electric pump and no power assist is felt, install a new Hydro-Max booster assembly and retest the system. If power is not available at the electric pump, correct the electrical concern and retest the system.