The boiler safety valve discharges through the red line, and potable water lines in the area drain through that white line. Cycle the 1/4 turn drain valve under pressure to try flush. Doesn't leak with pump, maybe due to lower pressure. Your built in regulator for city water supposedly set for 60 psi. How's the difference at the tap for city vs pump?
Lots of good info on the 17 Alde system in Much To Do About Alde.
James Mileur, HY80-2-Al,
2017 Classic Twin, 2016 RAM 3500 Megacab, ProPride hitch