best to do stone cold to determine that pressure gage holds for some time (1 hr or greater ) at pumped pressure. Run test at 1.2 bar or about 18psi so that you do not over pressure the system. If test run hot the test pressure will lower in time as the fluid contracts during cooling down and thus give a leak misread on holding pressure for an extended time.
BTW I just tested the coolant voltage to ground with digital mm and it is 16.1 mv or 0.0161v (about 10 min to steady due to sensitivity). I would look for voltage leak into coolant system. Check all grounds, batt connections and clean as needed. Use dielectric grease B4 securing on all conections and batt posts and their cable clamps .
