A breather assembly for a peristaltic pump comprising: a breather tube (12, 14); a cap detachably connected to the breather tube and comprising a sealing portion; and characterized in that: one of the breather tube and the cap comprise a guide track and the other of the breather tube and the cap comprises a protrusion which engages the guide track; the guide track comprises in series a first section and a second section which is separated from the first section by a first formation (76a, 76b) and is bounded at its distal end by a second formation; the protrusion is able to pass the first formation only when a predetermined first force is applied to the cap and the protrusion is able to pass the second formation only when a predetermined second force is applied to the cap such that the first and second formations prevent free movement of the protrusion along the guide track; and when the protrusion is located within the first section, the sealing portion of the cap seals against the breather tube, and, when the protrusion is located within the second section, the sealing portion of the cap is spaced from the breather tube to allow fluid to pass out of the breather tube. a breather tube (12, 14); a cap detachably connected to the breather tube and comprising a sealing portion; and characterized in that: one of the breather tube and the cap comprise a guide track and the other of the breather tube and the cap comprises a protrusion which engages the guide track; the guide track comprises in series a first section and a second section which is separated from the first section by a first formation (76a, 76b) and is bounded at its distal end by a second formation; the protrusion is able to pass the first formation only when a predetermined first force is applied to the cap and the protrusion is able to pass the second formation only when a predetermined second force is applied to the cap such that the first and second formations prevent free movement of the protrusion along the guide track; and when the protrusion is located within the first section, the sealing portion of the cap seals against the breather tube, and, when the protrusion is located within the second section, the sealing portion of the cap is spaced from the breather tube to allow fluid to pass out of the breather tube. one of the breather tube and the cap comprise a guide track and the other of the breather tube and the cap comprises a protrusion which engages the guide track; the guide track comprises in series a first section and a second section which is separated from the first section by a first formation (76a, 76b) and is bounded at its distal end by a second formation; the protrusion is able to pass the first formation only when a predetermined first force is applied to the cap and the protrusion is able to pass the second formation only when a predetermined second force is applied to the cap such that the first and second formations prevent free movement of the protrusion along the guide track; and when the protrusion is located within the first section, the sealing portion of the cap seals against the breather tube, and, when the protrusion is located within the second section, the sealing portion of the cap is spaced from the breather tube to allow fluid to pass out of the breather tube.