An apparatus for dosing a fluid flow system with treatment fluid/chemicals is described. The apparatus includes a vessel； an inlet pipe or hose and an outlet pipe for coupling to the system so that fluid from the system may flow into the vessel and return into the system to flush chemicals from within the vessel and into the system. The vessel has a piston within it and the vessel wall defines a cylinder in which the piston moves under pressure of incoming fluid from a first position at which the vessel contains fluid to be dispensed into the system to a second position corresponding to substantially complete emptying of the vessel. The piston has a bypass passage through or around it that opens when the piston substantially reaches the second position so that fluid from the system may flow passed the piston when the piston is at the second position.
