30 between surface 34 and washer 36 the operation of the valve will now be described the valve is normally coupled to a t piece connection in the pipe line on either the inlet or the exit side of a pump not shown with the valve fitted e g to the pump inlet side air which is trapped in the pump line ahead of any liquid from a tanker can be expelled through the valve during priming of the pump the trapped air will act on washer 16 and valve member 18 to overcome the spring bias and allow air to escape through ports 28 and 14 as shown by the arrows in fig3 as the air is removed washer 16 will sealingly engage over ports 14 to close off the valve the rising of the liquid will also cause float 40 to abut washer 26 and seal off ports 28 as shown in fig4 in this position the pump will have been primed and the pump will commence its normal pumping operation the pressure drop from pumping may allow float 40 to fall back onto washer 36 as the atmospheric pressure acting on valve member 18 is greater than the pressure inside the pipe line valve member 18 and washer 16 will prevent entry of air into the valve during pumping fig5 shows the main member 12 incorporated into a screw threaded member 42 which may be screwed onto a threaded t piece member 44 the member 44 joins the pump line not shown at right angles thereto a suitable seal 46 ensures a leakproof fitting if desired the threads could be replaced by flanges which are clamped together it will be appreciated.