Sorry to bump an old thread, but since I relied on the advice here when building a tap system, I thought I'd share my experience and some lab data in case it helps other people.
I built a 2 tap system, one for beer and one for carbonated water. I split the gas between the two systems using an inexpensive aluminum manifold with brass fittings from a major online retailer of brewing hardware. Everything in contact with water was separated by check valves from brass ... except the manifold which had a tiny area of brass in the check valve itself (the ball). To be safe I put it a few feet above the water level in the system and behind a lot of tube figuring the gas flow and gravity would be fine for a tiny bit of brass since it would never get wet.
Eventually I had doubts, so I flushed water through the tap and sent to an EPA certified lab for metal testing. Both copper and lead came back below EPA limits, but only barely, while my source tap water had undetectable levels of both. The lab recommended remediation. I swapped out that one little bit of brass (which was not even in contact with liquid) for stainless steel, sent a new sample to the lab, and the metal contamination went away. As far as I can tell, the water vapor alone was sufficient to liberate lead from the brass, which then was carried/blown into the drink by the CO2 flow. At least I am certain the valve itself never came into contact with liquid water.
I think I probably could have just kept drinking the lead contaminated water since it was technically below the EPA limit and probably been fine, although I would not want my kids drinking it (less of a problem for brewing beer). On the other hand, I paid $40 for a high quality, all stainless check valve vs. $200 for lab testing, so I should have have just put the check valve in from the beginning, known it was safe and saved all that money.
