The present invention relates to a brewing unit for a coffee machine. The brewing unit comprises a brewing chamber in which a dose of coffee powder can be put, an inlet for supplying water to the chamber for preparing a coffee beverage, a first piston providing a moveable wall for adjusting the chamber internal volume, which wall has an outlet connected to a spout for delivering the coffee beverage to the user, and a valve assembly for connecting said spout to the brewing chamber or the atmosphere according to the position of the supporting means. Advantageously a second piston is provided which moves the first piston and supports the valve assembly. The second piston is also moveable with respect to the first piston, for actuating the valve assembly.