I have a 4 tap keezer that where the CO2 is dispensed from a 4 valve manifold to each keg. The the first half of the keg generally pours really well leaving a nice head on the top of the pint. However, as the keg gets lower and lower my head grows and grows. Sometimes 2/3 of the pint is foam. Being as my other kegs are either carbonating or flowing nicely, I don't want to mess with my pressure. Is this common, or is there any way to stop this from happening?