Siphon assembly (24) includes a downleg (26) and an upleg (28) coupled at their upper ends to form an inverted U siphon with a piston and cylinder siphonic action flushing device (30) at a lower end of upleg (28). A threaded spigot (36) at the lower end of downleg (26) is sized to secure the assembly in a bottom opening of a cistern base (20), typically in substitution for a valve operated flushing device. An internal sleeve (44) provides a bore along the full length of downleg (26) which is sized for optimum flushing action.