Can we pressurize lunar lava tubes?

A lunar base could park or build pressurized habitats inside a lava tube but it would be simpler and much more useful to segmentize the tube and pressurize the entire interior. Regolith contains lots of oxygen (around 45% by weight) and water ice, of course, can be separated into hydrogen and oxygen. So producing the desired volume of atmosphere should not be insurmountable. Once pressurized, the atmosphere would be monitored, filtered and CO2 removed similar to a submarine. The question is, can a tube be air-tight? 

Lunar tubes are composed of basalt, which is very dense (a little denser than granite). It could be airtight- unless it is cracked or broken. The Moon has had and still has some tectonic activity (moonquakes) but not nearly as severe as Earth. Minor cracking could be sealed by spraying the area with an air-tight polymer. No one really knows what the inside of a lunar lava tube looks like - yet.