A priming valve works according to a simple principle mastered thousands of years ago, but perfected by the best of todays manufacturers of priming valves and similar equipment, such as compressors and vacuum pump priming systems. In short, a priming valve is merely a float mechanism that rises and falls in tandem with the level of a liquid inside the valves body. When there is no liquid in the body, the valve is considered to be in the open position. A passage is created from all inlet connections through to the vacuum outlet. In contrast, the priming valve is in the closed position when enough water has entered the body to cause the float to rise and close the passage.