Abstract: A steam trap allowing the escape of condensate from a working fluid of steam comprises an escape valve mounted in a chamber in which the working fluid flows. The escape valve includes a valve seat member and a deformable metal diaphragm normally closing the steam trap and preventing the escape of steam. The diaphragm closes a housing in which a control fluid of water initially only partially fills the housing to which a vacuum is later applied. The housing bears along the periphery of the diaphragm where the diaphragm is supported by a shoulder in the valve seat member. The valve seat member includes a cylindrical central chamber radially inwardly of the valve seat and an annular chamber radially outwardly of the valve seat. The cylindrical chamber is larger in diameter than and upstream of an escape passageway for carrying away condensate also in the valve seat member. The annular chamber is in communication through a check valve and passages with the chamber in which the escape valve is mounted.