thus the piston 94 and a bearing surface 104 or sealing surface 104 may form the vented chamber 102 the sealing for the fluid flows is provided by the piston 94 against the element 92 and the shaft 96 against the sealing surface 104 relief 106 108 may be provided as appropriate thus manufacturing tolerances may be provided while binding is eliminated for example fastening may tend to warp and bind components in one embodiment the shaft 96 may be provided with a bumper 110 adapted to make contact with a diaphragm 60 against a face 71 of a piston 62 the bumper 110 may be adapted to fit a hollow portion 112 of the shaft 96 a shank 114 may fit into an aperture 116 in the hollow portion 112 of the shaft 96 accordingly the bumper 110 may be secured thereby to travel securely with the shaft 96 thus the bumper 110 may provide stress distribution abrasion resistance and the like so as to minimize any deleterious affect by the shaft 96 on the diaphragm 60 the shafts 96 may thereby follow the diaphragm 60 and piston 62 for detecting the end of the stroke of the piston 62 at the body 16 rather than at the head 22 threads 118 119 may be formed in the element 92 or body 92 of the pilot 30 of fig5 a shoulder 120 may be adapted to stop the element 92 at an appropriate location in the head 22 in one embodiment a face 122 may abut a corresponding base in the head 22 the wall 124 of the element 92 may be secured within a retainer 42 as illustrated.