For those of you who use Sankey kegs, do you have any issues with the beer going flat? I naturally carbonate under presser and I recently transferred a batch to a Sankey keg and it was flat when I poured one. Could it be because the spear is much bigger than the 1/4" transfer hose and it's allowing the beer to splash too much which results in a lot of foam? Also, how much head space is too much? Will a 5 gal batch work in a 15 gal keg?