in the art will appreciate that the use of the rupture disc for piston access is simply the preferred way and generally more accurate than relying exclusively on shearing a shear pin a pressure regulation valve can also be used for such selective access as well as a chemically responsive barrier that goes away in the presence of a predetermined substance or energy field temperature downhole or other well condition for example to move the sleeve burst or rupture discs 42 also can be replaced by any other pressure control plug known in the art such as those disclosed and taught in u s patent application ser no 13 286 775 filed nov 1 2011 entitled u201c frangible pressure control plug actuatable tool including plug and method thereof u201d which is hereby incorporated by reference in its entirety after burst disc 42 is ruptured lower chamber 58 is under absolute downhole pressure so wall flexure at that location is minimized even before burst disc 42 breaks the size of lower chamber 58 is sufficiently small to avoid sleeve wall flexing in that region the use of a large boss to support intermediate internal bore piston seals 52 also strengthens inner shifting sleeve 50 immediately below upper chamber 62 thus at least reducing flexing or bending that could put inner shifting sleeve 50 in a bind before it is fully shifted the slightly larger dimension of external rod piston seals 59 as compared to port isolation seals 45 that hold inner shifting sleeve 50 closed initially also.