What is causing my brake pedal to sink to the floor when I start my truck but when it's not running brake pedal is perfect.. please help 92 4runner 4x4 v6
When you start the engine, the vacuum boost is then applied and the vacuum booster helps you press the pedal down. With the engine off, you get no power assist, so the pedal feels "nice and firm". You are not specific if the pedal is OK when driving, or ONLY when starting. If only when starting, that's normal, because you go from no vacuum power assist, to having it. Now if pedal is mushy or goes to floor also when driving, then you need to check the brake system.
If you are saying that the pedal goes to the floor without pressing on it, then it sounds like your brake servo (booster) has a faulty valve. The servo uses the vacuum created by the engine to assist when pressing the brake. If one of the valves in the servo becomes faulty, the vacuum assists when not pressing the brake and pull the pedal down. When you turn the engine off, the vacuum releases and the pedal rises. You likely need a new brake servo.