Try below for the brake booster if it performs its job. Check the master cylinder this is where hydraulic pressure is created-Have you inspected the wheel cylinders on the back for leakage-If there's no actual leakage were right back to the master cylinder.
To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.
Vacuum boosters also have an external one-way check valve at the hose inlet that closes when the engine is either shut off or stalls. This traps vacuum inside the booster so it can still provide one or two power assisted stops until the engine is restarted. The valve also helps maintain vacuum when intake vacuum is low (when the engine is under load or is running at wide open throttle). You can check the valve by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.
April, 12, 2008 AT 11:00 AM
Bench bleed the master by getting brass plugs and screw them into the two ports, then pump the master until you cant move the piston any more. Attach the lines, Have an aide press and hold the pedal crack the lines and retighten, repeat this till all air is removed at the lines, Don't let the master run dry, bleed rear cylinders by opening the bleeders and wait until the fluid dripps out. Close bleeders. Make sure the rears are properly adjusted, then test the pedal hieght