Pressure, nothing. Good espresso requires the neopolitan style "upside-down" espresso maker. It's three pieces; from bottom up, a boiler, a filter, and the pot (which looks upside-down at this point). The whole thing screws together, the water is boiled over heat, then the whole unit is inverted allowing the *extremely* hot water to flow through the filter (and thus the grounds) very quickly. Pour into espresso cup and enjoy.
Bonus 1: my "medium" maker can make 1-4 shots at once.