transitional area 78 of means 70 couples cylindrical valve portion 74 to an integral piston 80 which includes a cylindrical body with a pair of annular slots 82 for receiving sealing rings 84 therein rings 84 seal the outer cylindrical portion of piston 80 to the cylindrical sidewall 86 of the piston chamber 88 formed downwardly in housing 56 as illustrated in fig1 assembly 72 also includes an integral stem 90 which extends upwardly from the center of piston 80 and sealably extends through the top 92 of the detector 50 by means of an o ring seal 94 fitted in an annular groove surrounding the cylindrical sides of stem 90 the piston chamber 88 includes a conically tapered passageway 89 at its lower end which communicates with conduit 63 a cylindrical passageway segment 91 extends from an end of tapered passageway 89 to a second conically tapered section 93 which in turn extends to the cylindrical valve seat 65 the diameters of the cylindrical sidewall 86 conically tapered segment 89 and cylindrical segment 91 are greater than the mean diameters of the conically tapered portion 78 of assembly 70 and the cylindrical valve 74 such that a lower piston chamber 100 is defined by the space.