located at the end of the rod 66 is provided to hold the head 82 of the piston 54 on the rod 66 some play is allowed between the rod 66 and the head 82 in order to allow sufficient leeway to accommodate some misalignment between the rod 66 and the remainder of the bleed valve 50 a spring 90 is positioned between the housing 52 and piston 54 the spring 90 abuts against the housing 52 between the flange 72 and the central portion 62 the spring 90 also abuts against the central portion of the piston 54 adjacent the aperture 86 for receiving the rod 66 the rod 66 carries a stop 92 on its end remote from the bolt 88 this stop 92 limits the movement of the rod 66 in an axial direction the bleed valve 50 described above can have any appropriate dimensions however tests have shown that a bleed valve 50 having a head portion 82 with a diameter of around 26 mm and a piston 54 with an outer diameter of around 49 mm operates effectively other dimensions will be selectable around these dimensions to suit particular applications and to achieve a compact design it is preferred that the rod 66 is allowed to move axially by at least 4 mm the bleed valve 50 is illustrated in a closed position the outer edges of the head 82 are pressed against the seal 78 so that the unit as a whole is closed port 56 is connected to the airflow path of the vacuum cleaner immediately upstream of the dust separating apparatus 16 or the downstream cyclone 16 b and therefore the upstream pressure acts directly on the surface of the piston 54 which is shown to the left in.