member such as that illustrated is moved axially as it is expanded the axial movement up the frustocone naturally causes the leading end of the seal element to contact the surface 18 first and begin to drag thereon because of the drag it is possible in prior art devices for the element to extrude over the backups 30 and thereby defeat its purpose in the invention the configuration of the seal element 22 is such that drag on the seal surface 40 is insufficient to cause material of element 22 building up enough to crest the backups 30 at end 36 of the member 10 rather the backups 30 at the trailing end of the member 10 which in the illustrated embodiment of fig1 is at end 36 of the element 22 will contact and conform to the surface 18 before the element 22 can drag over the backups 30 the step 42 then is of significant benefit to the art in that it appreciably enhances the ratio of elements set properly to those not set properly referring now to fig2 wherein similar or identical components are numbered using 100 series numbers of those used in connection with the fig1 embodiment an alternate embodiment of a sealing member 110 is illustrated in this embodiment there are two distinctions over the prior described embodiment of fig1 these are a step 142 on both ends of the element 122 and that the element 122 closely follows the contour of the recess 120 on both ends 136 and 138 of the element 122 this embodiment allows for simpler construction of the member 110 as the element 122 does not.