An infusion unit includes an infusion chamber for coffee or other product where, through water under pressure, flavors are extracted for making a beverage. The infusion chamber includes at least one yielding wall for adjusting a volume of the closed infusion chamber according to the amount of product contained into the infusion chamber. Moreover, a variable volume chamber is provided, which is filled with a liquid and connected to the yielding wall, so that a volume of the variable volume chamber filled with liquid varies according to a position taken by the yielding wall when the infusion chamber is closed. The position of the yielding wall is locked when the infusion chamber is closed, preventing the liquid flow from and into the variable volume chamber filled with liquid.