fig3 and 7 alternatively the helical passage can be formed on the inner surface of wall 33 further the passage may be labrynth type such as numerous 90 degree bends to offer increased resistance to fluid flow the plug 41 further includes a transverse wall 45 that includes a cylindrical wall portion 45a and a transverse wall portion 45b so that it defines a first chamber 46 and a second chamber 47 fig3 10 the helical path 44 provides a restricted flow between the chambers 46 47 transverse wall 45 includes spaced openings 45c and a central opening 45d through which the projection 48 of a mushroom one way or check valve 49 of resilient material extends the mushroom check valve 49 includes a flexible laterally extending portion 50 that normally covers the opening 46 and prevents flow from the first chamber 46 to the second chamber 47 but permits flow from the second chamber 47 through the opening 46 to the first chamber as shown in fig5 and 6 the ends of the frusto conical portion 42 of plug 41 are formed with circumferentially spaced radial grooves 52 53 that communicate with the helical passage 44 defining a plenum in association with the end walls and thereby providing communication between the chambers 46 and 47 filters 53a 53b are provided at the ends of the frusto conical portions to filter the passage of air the filters are kept out of contact with the end walls and particularly the axial openings 38 36 by appropriate means such as integral ribs 54 on end wall 32 and integral ring 55 and radially inwardly extending ribs 56 on end wall 34 fig8 the diameters of the walls 33 42 and.