Think about this for a minute. You have 1 atm absolute in the keg. You pressurize to 2 atm gauge which is 3 atm absolute with CO2. You have, then .2 atm O2, 0.8 atm N2 and 2 atm CO2. You bleed down to 1 atm which gives you 0.2/3 atm O2, 0.8/3 N2 and 2.0/3 CO2. Pressurize to 2 atm gauge again and bleed down again and you have 0.2/3/3 atm O2, 0.8/3/3 and the rest CO2. Do it again and its 0.2/3/3/3 = 0.007 atm i.e. 0.7% O2 still in the keg. To beat this some fill the keg with water and push that out with CO2. I fill the keg with steam in order to sterilize it and circulate that steam for 15 min (these are Sankeys). At the completion of sterilization the keg is disconnected under pressure from the steam source and connected to a CO2 bottle. As the steam condenses it is replaced by CO2. Excess CO2 then pushes the condensate out.