A filter device (7, 8) for a conduit (9, 10) comprises a gas-permeable pre-
filter element (21, 23) and a gas-permeable, liquid-impermeable
filter element (22, 24). The filter device (7, 8) is configured to be fluidly coupled to a conduit (9, 10) such that a gas can be caused to flow, in sequence, through the pre-
filter element (21, 23), the filter element (22, 24) and then the conduit (9, 10). Such a filter device (7, 8) can for example be used in each of a vehicle hollow driveshaft (2) and a hollow wheel
stub axle (6) forming a CV joint (11) having a
compressed air supply line which passes therethrough, as is the case in some central tyre inflation systems (CTISs). Thus,
compressed air can be supplied via said air supply line without the need for an additional fluid transfer device fluidly connecting the driveshaft and the
stub axle's conduits (9, 10).