Well, distilled or deionized water will absorb metal ions and deterioration of the metal. That is why metal tubing is almost never used on Reverse Osmosis sytems etc.
Deionized water is exactly that - water that has essentially been stripped of all of its ions. Water likes to be balanced in its natural state, however, and this means that it adds ions to itself to achieve that goal. Therefore, DI water grabs ions from everything it touches that can be dissolved or absorbed. It is about a close as you can get to a Universal Solvent. In your case, it will extract metals from all of the brass fittings you have, and will also pull carbon dioxide from the air - you get the drift.