Abstract: A movable wall assembly for use in a servomotor having a resilient diaphragm with a bead radially compressed in a groove on a hub by a backing plate to form a seal between a vacuum chamber and a power chamber. The diaphragm correspondingly axially urges the wall assembly into contact with the hub to prevent relative movement therebetween.

Abstract: A control unit which is particularly useful in controlling the output pressure of a turbocharger in response to the manifold pressure with compensation for pressure drop at the throttle characterized by a reference unit disposed in the housing for the manifold pressure to act on and at least one diaphragm extending across said housing to apply the pressure differential between the manifold pressure and the output pressure of the turbocharger. In one embodiment, the reference unit includes a bellows of a predetermined volume of gas which schedules the displacement of the reference unit in response to the temperatures of the gases of the manifold. Another embodiment includes an additional diaphragm and valve means for applying additional pressures to the reference means to compensate for the altitude for the system to prevent overspeeding of the turbocharger.