I force-carbonate all of my beer in kags. Once a beer reaches the optimal level of carbonation, if Ireduce the CO2 pressure down to, say, 8-10 PSI for serving, will the beer loose carbonation over time?
As long as there is no leak in a keg of beer in which the CO2 saturation has stabilized, there will be no loss of pressure over time. That said, there are a number of things in your question that suggest you may see a change in level of carbonation.
You are apparently force carbonating at high pressure (25-30 psi?). Are you consulting a pressure/temperature chart to figure the proper pressure at the temperature you have the beer in order to get the desired level of carbonation? Using these charts, you can carbonate properly at just about any reasonable temperature. I carbonate at around 65Â°. Done with the charts, the carbonation level will remain the same when you chill it down to serving temperature. You just need to remember to not hook up the gas at the lower pressure until after the beer temperature has stabilized at serving temperature. If you bleed off the high pressure gas before the temperature has dropped, you will lose some carbonation as the beer needs to absorb the excess pressure from the head space. If you don't have a backflow valve in place, hooking up the lower pressure gas to a keg at higher pressure will result in beer backing up into your gas line and possibly into your regulator.
Also, you need to make sure your kegging system is balanced. There is much more involved than just turning your gas pressure down to 8-10 psi for serving. You need to consider the volumes of gas in the beer (carbonation level), temperature, and length of your serving line. The pressure of the CO2 applied to the keg to maintain carbonation level is determined by the temperature and desired carbonation level. The serving pressure is then determined by the length of your beer line. The longer the beer line, the more resistence to flow you will have. If you have the line too short, all you will get is foam from the beer gushing out too fast. Once the foam settles the beer will be flat. If the line is too long, the carbonation will be perfect but the flow may be too slow to fill the glass in a reasonable time. In most cases 5'-6' of 3/16" I.D. beer line will be just about right. If you are using a larger diameter line, such as 1/4" I.D., you will need a much longer line, perhaps up to 20'.
To help you figure all this out check out this link:http://hbd.org/clubs/franklin/public_ht ... lance.html