Minecraft wiki entry for Basic_Piston_CSG, associated text: The second repeater must be set to at least delay 2, or the piston will drop its switch block. If this happens, the generator can be restarted with the button. If the generator fills its pillar, it will stop until the newest cobblestone is cleared. Besides mining it directly, this can be done by mining any block(s) in the pillar and pressing the button. The pale stone brick indicates a few optional blocks below the fluids.