exposed to the high pressure from the resin input port 80 when the mono valve is in the closed position e g the ball 27 is seated in the ball valve seat 84 referring to fig4 and 5 cross sectional views of a catalyst air valve dual input valve in a closed position fig4 and in an open position fig5 are shown catalyst enters the gun head assembly 31 from the catalyst fitting 17 through a catalyst input port 90 into an area on the pressure side of the catalyst valve 97 formed on the dual input valve stem 4 the pressure of the catalyst and the bias of the dual input valve spring 36 hold the catalyst valve 97 against the catalyst valve seat 94 as shown in fig4 preventing flow of catalyst from the catalyst input port 90 through to the mixing chamber 100 an optional o ring 51 further enhances this seal compressed air enters the gun head assembly 31 from the compressed air fitting 5 through a compressed air input port 98 into an area on the upper pressure side of the dual input valve stem 4 the pressure of the catalyst and the bias of the dual input valve spring 36 hold the compressed air valve 99 of the dual input valve stem 4 and seated o.