I'm not sure, I know that copper can leech into pure water, but I couldn't tell you if brass does. You can get a plastic version of the Y piece from B&Q, so if you're concerned about it get a plastic one. I haven't found any probs with my setup, but then again, I have my di vessel as the last thing before the hose feeding the pole so if the brass Y piece is raising the tds I wouldnt know it.
Tim