An automatic draining back flow prevention apparatus comprises tubular body structure having main passage structure between flow entrance and exit ports; the body structure having first and second side ports communicating with the passage structure; first and second diaphragms carried by the body structure to be exposed to flow in the passage structure; a stopper in the passage structure cooperating with the first diaphragm to pass forward fluid flow while a first diaphragm flexes to block exit flow of fluid through the first side ports, and to block back flow of fluid through the main passage structure when the first diaphragm moves to unblock exit flow of fluid through the first side port; the second diaphragm moving to block exit flow through the second side port in response to the forward flow of fluid through the main passage structure, and being movable to allow drainage flow of fluid through the second side port during at least one of the following conditions: when the stopper and first diaphragm block back flow of fluid through the main passage structure, and when the stopper and first diaphragm are at rest.