The screen in between the filter and the chamber top is to prevent grounds from entering up into the chamber, but boiling pressurized water absolutely does flood the entire chamber including the plastic region. (It's not just a wall of pressurized water in nice straight lines, it's a mixture of solids liquids and gasses.) Eg: When I clean my E61 group head, I have to remove a fine mesh filter to get to the brass outflow fitting, and it is still covered with oils from the coffee.