Coffee brewing apparatus

Full automatic coffee machine comprising a coffee extraction unit, said coffee extraction unit comprising:

• a brewing chamber for receiving roasted and ground coffee powder, said chamber comprising an upper rim, a bottom wall and a water inlet,
• an upper piston,
• said upper piston being configured to be movable through the upper rim of the brewing chamber and inside the brewing chamber further to the relative movement of the brewing chamber and the upper piston, and
• said upper piston comprising a liquid outlet duct,
• a lower piston movable inside said brewing chamber between:
• a lowest position wherein the lower piston lies on the bottom wall of the brewing chamber, and
• a highest position wherein the lower piston is at the upper rim of the brewing chamber,
• the lower piston comprises a piston rod, said piston rod extending through the bottom wall of the brewing chamber, and
• the piston rod comprises an internal channel extending from a channel inlet at the lateral wall surface of the rod to a channel outlet at the bottom wall surface of the rod, and
• the channel inlet is designed and positioned on the lateral wall surface of the rod so that said channel inlet overlaps the bottom wall of the brewing chamber only when the lower piston is in the higher position.