A self-venting spool valve assembly includes a novel mechanism for retention of a rotatable spool within a longitudinally disposed valve body. The spool contains first and second flanges at opposed ends thereof, wherein the first flange is adapted to bear against the longitudinal body of the valve assembly. The second flange, however, is spaced from the valve body by a groove, and a biasing retainer is disposed within the groove for frictional engagement between the second flange and the valve body in order to sealingly retain the spool within the valve body. In the preferred form, the spool and associated bore are tapered, and contain fluid drain and air vent apertures for selective communication with fluid drain and air vent passages extending through the longitudinal body of the valve assembly.