A vacuum-operable unit for use with an ignition distributor in which a casing is divided into first and second chambers by a resilient diaphragm. A plunger is slidably received in the first chamber, with the plunger being secured at one end to the diaphragm and extending at its other end from the first chamber for connection to a distributor. Axial movement of the plunger varies the ignition timing of the distributor. First and second conduits are provided in communication with the first and second chambers respectively, and resilient means within one of the chambers resists movement of the diaphragm in a direction to reduce the volume of that chamber. Sealing means acts between the wall of the first chamber and the plunger to fill the first chamber while allowing relative movement between the plunger and the wall of the first chamber, with the sealing means being constituted by a pair of annular members secured to the plunger and the wall of the chamber respectively, and interconnected by a flexible tubular member. The diameter of the flexible tubular member of the sealing means tapers from a maximum diameter adjacent one annular member to a minimum diameter adjacent of the other annular members and the tubular member is turned inside out intermediate its ends so that part of the tubular member surrounds a further part of the member, and the tubular member together with said annular members constitutes a rolling seal.