Abstract: A gas vent orifice (78, 148) is located within a gas vent passageway. A closure ball (80, 154) is located upstream of the orifice (78, 148). A piston (66, 166) is located downstream of the orifice (78, 148). The piston (66, 166) includes a projection (76, 170) which projects into the orifice (78, 148), for dislodging the closure ball (80, 154) from its seated position. A mass in the form of a spherical ball (74, 160) functions to move the piston (66, 166) to dislodge the closure ball (80, 154). The mass (80, 154) rests on a conical ramp (56, 126) and the slope of the ramp serves to direct the mass (80, 154) against the piston (66, 166). A steep sloped ramp (126) may be used in combination with a piston (166) having a cylindrical center portion (168). Or, a shallow ramp (56) may be used in combination with a piston (66) having a flat upper surface and radial legs (68, 70) which fit within radial slots (58) in the ramp (56).