My Ford F250 4x4 brake pedal goes to the floor when you start the truck and stay's to the floor until you shut the engine off then it returns? The brake booster, master cyl, rotors, pads, brake lines, drums, brake shoes, and brake cylinders have all been replaced.
One of the valves in the brake servo (booster) is stuck open. When you start the engine the vacuum produced by the engine (or a separate vacuum pump) is used to assist braking pressure from your foot. As you press the brake pedal, a valve in the servo opens to allow the vacuum to pull against the diaphragm in the servo to assist you. If a valve is stuck open it will keep sucking until the pedal goes to the floor and will only release once the engine has stopped.
Presumably it is putting the brakes on when this happens. If this is true, it is nothing to do with bleeding.
Depending on the design of the servo, there may be a pipe that connects the servo casing to the servo valve block, if this is leaking or missing, the same symptoms would occur. Any air leak into the casing in the rear of the servo body will also produce these symptoms.