some circumstances the combination of temperature and the pressure differential created between the vacuum in chambers 60 and 62 and air at atmospheric pressure is such that shell 32 deflects toward shell 34 however face or head 112 engages rib 116 to limit movement of the shell 32 toward shell 34 to a specific fixed spatial distance since strut 98 only sees compression forces it is designed to be of such size to not substantially increase the weight of the power assist apparatus 18 it is envisioned that the strut 98 could be made of plastic and yet perform the function of maintaining the shells 32 and 34 at the minimum spaced relationship when the under hood temperature decreases the plasticity of the material decreases and as the temperature is reduced to below 120 u00b0 f the material returns to a substantially rigid structure thus the movable wall 36 is provided with the capability of moving a maximum distance toward shell 32 without being hampered by the engagement of hub 50 on a deflected rib 94 in a secondary embodiment shown in fig2 wherein like parts are used the same reference numeral is also used in this embodiment master cylinder 211 is fixed to the spring tower support 212 by fasteners or bolts 214 and 215 the flange 216 of master cylinder 211 has opening 218 through which bolts 220 extend to attach housing 232 of the power assist apparatus 219 shell 232 is.