The present invention refers to a flush tank providing a receptacle with a discharge valve and a triggering mechanism therefore. A first end of an adapter (5) is inserted with a press fit into a discharge projection (3) of the receptacle (2) for the flush water. A discharge tube (7) to discharge the flush water from said receptacle (2) is attached to a second end of said adapter (5), wherein a triggering mechanism (8) is arranged within said tube (7), said mechanism releasing flow of the water out of said receptacle (2). The triggering mechanism comprises a body (16) in which a first hollow piston (22) is slidably arranged whereby lifting the first piston initiates a small flush. A second piston (25) is in turn slidably arranged in the first piston whereby lifting the second piston initiates a large flush. The pistons are both actuated pneumatically.