A pulsator (10) comprises a valve chamber (12) and an armature (24a) arranged in the valve chamber. An electrically powered coil (20a) at least partially surrounds the valve chamber and generates a magnetic flux. A magnetic pole piece (22a) is arranged in the valve chamber (12) and has a metal surface (23a). The armature (24a) has a first end (26a) nearest the magnetic pole piece and a second end (28a). The first end of the armature has a metal surface (30a) and the second end has a first sealing surface (32a) and a second sealing surface (34a).