The suction line (23) in the first endface (21c) of the rotary slide valve (21) facing the divider disc (11) has an outlet orifice (23b) and connects via the connector (11a) to the cylinder (16) in agreement with the turn angle of the slide valve when the piston (17) is aspiring. The slide chamber (20) element (20b) pre-tensions the slide onto the disc so as to clamp the endface to the disc. The endface is pref. flat and the piston has two pistons for respective cylinder bores (16). The disc connector has two intakes for the respective bores to admit air from suction chamber (3b) via line (23). The endface is grooved (24) to link up the suction connections when one piston is at the end of its displacement stroke and the other in the compression stroke. Opposing endface sectors defined by the groove and the two suction connections are assisted by three groove sectors, the first two radial to the slide centre (21) as against the third groove sector used as link between the first two, with the first slide endface sector enclosed by the three groove sectors on three sides.
