I'll tell you how I do this on larger engines,the secret is to drop the oil pan which is hard to do on smaller trucks but is easier in big rigs and most construction eq. I leave everything connected on the engine and the coolant level full. Drop the pan and raise the pressure as high as the system will permit (usualy 15 lbs) and watch for leaks from below, even small ones show up pretty well -if this does not work go to a blacklight type dye and turn off the lights and do it again. I have found many small leaks over the last 30 years and several that the dealers had given up on.