I'm assuming the system was flushed after the core replacement?
The sludge could be a couple of things. The failed core and presense of sludge, usually indicate 2 things to me. Either 1) There is or was a external leak in the sysytem someone added a bunch of stp (barz) leak in the system to plug up the leak, and ended up plugging the core at the same time. Or 2) the head gasket failed because the core usuall gets the oil/coolant mix plugged up in it.
The voltage reading could just be the fact that the core is copper (conducter) and it's redidule voltage from the dashboard electroincs passing through it to ground
