When I bought my Brno from Mays gun shop, they recommended I put the pistol in the freezer (!) for a while before bulk filling it - put it in a cloth and then a plastic bag first. The reason is the storage reservoir needs to be cold to get a good charge in there. Then I found out that just filling it a bit and then firing ten shots rapidly would do the same thing. Also, yes you do have to let the liquid co2 flow into the reservoir. You have to let the pistol warm to room temperature after, but it doesnt take long.
So, fill it a little, then dry-fire ten shots, then fill it properly. You can't over-fill with CO2 as far as I know, it runs much much less pressure than HPA as it liquefies at a much lower temperature (CO2 molecules like sticking to eachother more than the mix of gasses in air, and is therefore more viscous). It is a pretty safe propellant, and you cannot have the kind of catastrophic explosions that can happen with HPA. The downside is if you fire alot of rounds rapidly on a cold day, not only will the power be very much lower than if it was powered by HPA, but there is a risk of finely engineered parts getting supercooled and jamming. My brief relationship with a QB78 included a jammed bolt during an accuracy testing day in very cold weather, and that rifle had been smoothed out by an expert.
Have fun!