An automatic vacuum-relief apparatus for liquid suction systems includes a vessel defining a chamber filled with liquid in communication with a liquid flow line, a rotary shaft extending through the chamber to support a valve device adjacent to the liquid flow line and to move the valve device to open a relief-air flow channel to relieve increased suction caused by inlet blockage, and a piston on the rotary shaft responsive to suction increases within the liquid flow line and chamber to actuate shaft rotation. The valve device, chamber and relief-air flow channel are configured and arranged to cause a spurt of liquid from the chamber into the inlet while the valve device is moving from its isolating position toward its opening position upon inlet blockage, whereby the spurt of liquid pushes the blockage away from the inlet and the relief air reduces the suction. The valve includes a liquid-flow barrier secured with respect to the liquid flow line such that the barrier is in an open position allowing flow of liquid from the inlet to the outlet during normal operation and in a closed position isolating the inlet from the outlet in response to suction increases upon inlet blockage.