A brewing unit 1 for a coffee machine comprises a brewing cylinder 2 and a piston 5, 13, wherein the brewing cylinder 2 and the piston 5, 13 are movable relative to one another in the axial direction. An activatable seal 12, 18 or seal assembly is provided to seal the piston 5, 13 against the inner wall 3, 15 of the brewing cylinder 2. The seal 12, 18 is activated by an axial movement of piston 5, 13 with respect to the brewing cylinder 2. Under compression when activated, to form a seal between piston 5, 13 and brewing cylinder inner wall 3, 15, and which seal 12, 18 or seal assembly. When not activated, the seal 12, 19 is either not at all, only under low compression contact with the element.