I bought my 2004 with 250K miles on it. I had to replace the rear caliper which had a frozen bleeder screw and then bled the brakes by pumping. When the van is turned off the brake pedal holds firm so I assume the master cylinder is OK. Start the engine and the pedal goes to the floor. Pump a few times and it will firm up but still slowly go to the floor with pressure. I checked the power booster check valve and it holds pressure. Vacuum to the valve seems good but I hear a slight hissing when the valve is connected to the booster. Is the power booster shot?