When pressure test was performed, did the pressure drop indicating a leak? Was it performed with a cold or hot engine?
Leaks that are minor might take a long time to show a drop in pressure and sometimes you might not see anything indicating a leak.
Around areas of possible leaks, look for signs of accumulation of coolant residues (powdered residues). These most commonly occurs at hose joints, water pump, radiator ( by the way look at the drain cork closely) If there is a leak there, it would not leave any stains.
As I mentioned, if coolant is depleting, there must be a leak somewhere, only you have not been able to locate it yet.
Monday, October 30th, 2017 AT 9:32 AM