20 the member 20 is thereby placed under compression and as a result imposes a closing force between the end 22 of the member 20 and the nozzle 10 the degree of flexing is preferably predetermined to impose a suitable closing force between the end 22 of the member 20 and the nozzle 10 fig7 shows the end wall 21 having an outwardly bowed configuration when in the unstressed condition that is not essential by way of example the wall 21 could be flat as shown by fig9 when in the unstressed condition the effectiveness of the seal between the stem end 22 and the nozzle 10 may be enhanced by providing sealing means such as resilient sealing means at the end 22 alternatively as shown by fig7 and 8 the sealing means may include a relatively thin flexible circular sealing lip 23 extending generally in the axial direction of the member 20 other types of sealing means could be adopted also the sealing means may be provided on the nozzle 10 rather than the member 20 or may be provided on both the member 20 and the nozzle 10 fig8 illustrates a sealing arrangement between the member 20 and the nozzle 10 that has been found satisfactory in practice as shown the end 22 of the member 20 is.