Abstract: A safety lock cap for use with a cardiotomy reservoir, having a passageway formed therein under control of a positive pressure relief valve and projecting axially into the reservoir beyond the outer tubular portion of the cap inserted into a port opening in the reservoir vessel. Under positive pressure, the cardiotomy reservoir is vented through the passageway so that during operation of a heart-lung machine the safety lock cap will provide a minimum amount of safe positive pressure and become a relief valve that decompresses the cardiotomy reservoir.