Improved drainage valve

A drainage valve (10) for use in pressure systems where pressure surges occur for draining an underground water system when the water pressure in the underground water system is off and for sealing the drainage valve against drainage when the water pressure is on comprising a housing (11) having a first annular support (24) and sealing surface and a second annular support (56) and sealing surface for supporting a member (14, 51) that is nonextrudable through a drain passage (22, 55) in the drainage valve under normal operating pressures and surge pressures encountered in water supply systems and to permit said nonextrudable member to open said drain passage when the water pressure is off.