10 and the thrust pin 38 wherein the first valve 26 is closed while the second valve 32 is open fig1 b on the other hand illustrates an intermediate position wherein the thrust pin 38 has been forced upwardly in order to contact the ball 42 while simultaneously closing the second valve 32 lastly fig1 c exemplifies the invention upon the thrust pin 38 having reached a fully extended second position whereby the first valve is fully open while the second valve is closed as best shown in fig2 the first valve 26 includes a ball 42 located in the upper chamber 22 that when in the closed position as illustrated simultaneously sealingly engages a first valve seat 44 machined into the body 16 and an elastomeric sealing element 46 preferably an o ring as is evident in fig2 when the first valve 26 is in its closed position there is a space 48 created between an upper shaft portion 50 of the thrust pin 38 and the ball 42 thus the supply pressure from the supply gas maintains the ball 42 in its engaged position the improvement provided by the present invention encompasses the use of an elastomeric seal within the first valve 26 while maintaining metal to metal contact between the ball 42 and the valve seat 44 in this regard the valve seat 44 is provided with a recess 52 of the depth sufficient so that the o ring 46 may be self restrained therein and exposed only a small degree such that when the first valve 26 is in its closed position the ball 42 simultaneously sealingly engages both a surface portion of the valve seat 44 and the o ring.