These improvements consist in the use of a highly elastic membrane (13) as a valve element, fixed by means of flanges 14 and 17 to a support plate 15, conforming an annular chamber 16 which may be pressurized through the conduit 19 to deform membrane 13 towards a closure situation where same falls upon the valve seat 24, an opening being established in the centre of the membrane-support plate assembly to which is axially coupled an outlet conduit 22 for release or outlet of the residual air present in the diffuser 9.