A pressure pack dispenser for dispensing a product comprising: a container for dispensing a product therefrom, the container comprising a cylindrical container wall, a lower end wall, and a product outlet through which the product is dispensed, and a composite piston movably mounted within the container and adapted to move from a first position adjacent the lower end wall to a second position adjacent the outlet, the composite piston comprising a first upper piston, a second lower piston and a coupling means (411, 412), the coupling means movably coupling the first and second pistons to each other and permitting limited relative axial movement between the first and second pistons, the axial movement being in a substantially longitudinal direction parallel to the direction of movement of the composite piston, the container wall and the first upper piston defining a product chamber within the container, whereby movement of the composite piston within the container towards the outlet expels product through the outlet, the container wall and the second lower piston defining a propellant chamber containing a pressurised propellant, wherein the composite piston comprises a viscous sealant material located in a piston sealant chamber between the first and second pistons and adapted to be forced into engagement with the container wall by a compression force which acts between the first and second pistons to cause the second piston to move towards the first piston, characterised in that: the pressurised propellant comprises a compressed gas which remains in the gaseous phase as the piston moves from the first position to the second position, and a clearance is provided between the second lower piston and the container wall which is filled with the viscous sealant material during movement of the composite piston within the container towards the outlet, the viscosity of the viscous sealant material and clearance being selected such that a thin film of viscous sealant material is left on the container wall of the propellant chamber as the composite piston moves within the container towards the outlet, thereby reducing the volume of viscous sealant material in the piston sealant chamber and reducing the friction force between the composite piston and the container wall. a container for dispensing a product therefrom, the container comprising a cylindrical container wall, a lower end wall, and a product outlet through which the product is dispensed, and a composite piston movably mounted within the container and adapted to move from a first position adjacent the lower end wall to a second position adjacent the outlet, the composite piston comprising a first upper piston, a second lower piston and a coupling means (411, 412), the coupling means movably coupling the first and second pistons to each other and permitting limited relative axial movement between the first and second pistons, the axial movement being in a substantially longitudinal direction parallel to the direction of movement of the composite piston, the container wall and the first upper piston defining a product chamber within the container, whereby movement of the composite piston within the container towards the outlet expels product through the outlet, the container wall and the second lower piston defining a propellant chamber containing a pressurised propellant, wherein the composite piston comprises a viscous sealant material located in a piston sealant chamber between the first and second pistons and adapted to be forced into engagement with the container wall by a compression force which acts between the first and second pistons to cause the second piston to move towards the first piston, the pressurised propellant comprises a compressed gas which remains in the gaseous phase as the piston moves from the first position to the second position, and a clearance is provided between the second lower piston and the container wall which is filled with the viscous sealant material during movement of the composite piston within the container towards the outlet, the viscosity of the viscous sealant material and clearance being selected such that a thin film of viscous sealant material is left on the container wall of the propellant chamber as the composite piston moves within the container towards the outlet, thereby reducing the volume of viscous sealant material in the piston sealant chamber and reducing the friction force between the composite piston and the container wall.