Every 6 months, I flush my 7 corny system's gas manifold and gas lines with PBW, water, Star San, then co2. The lines and regulators do get beer in them even with the check valves. The system doesn't like pressure changes from adding cornys that were force carbonated. Even after you bleed the excess pressure off, the beer continues to give up its co2 and the beer will slowly move backwards. And to compound this by pulling the regulator pressure relief ring will move beer backward quickly. The check valves are eventually useless after beer has gone through them. Beer is sticky.
Life is simple if you are set up with MFL fittings on your system. Just stick your corny with cleaning agent between the regulator and distribution block, disconnect all the gas quick connects from the corny. Push the cleaning agent through the lines with co2, bleeding the agent through the lines by loosening the MFL gas quick connects. Let set overnight. You can flush the lines individually by removing the quick disconnects one at a time, running the agent into a bucket. Follow with a water, then sanitizer flush. Finally, follow by co2 flush to remove moisture.
I now force carbonate and filter from a separate tank and regulator.
If you use PBW make sure it is totally dissolved into solution.
Most regulators disassemble easily to clean the diaphragm.
Gary
BN Army // 13th Mountain Division 
