There are a few things you can check and fix to prevent your moka pot from leak and overflow - Fill water in the bottom chamber just below the pressure valve. If you fill more water, the excess may be draining out from the pressure valve. - The pressure valve is clogged. Give the bottom chamber a thorough scrubbing with a bristle brush. - Rubber gasket isn't sealing properly. The rubber gasket in the upper chamber hardens over time and develops cracks. Replace the gasket and make sure you have a tight seal of the top and bottom chamber before your brew begins.