A seal for sealing an interface between a piston shaft (4) of an expander mechanism for a disk brake and an inner surface of a housing (5) of the brake in which the shaft is received is described. The seal comprises: a laterally outwardly disposed portion (11) adapted to engage a portion of the housing in fixed relation therewith; a laterally inwardly disposed portion (12) which is adapted to form a static seal with a surface of the shaft; and an intermediate portion (13) interconnecting said laterally outwardly disposed portion (11) and said laterally inwardly disposed portion (12) such that the laterally inwardly disposed portion is permitted to move axially together with the shaft relative to the laterally outwardly disposed portion, in use, in static sealing relationship with the shaft.