We had one of our engines leak into another via the common maintenance tank fill and drain loops. We had the engine we were to work on drained and isolated. Pulled a prechamber valve and water started pouring in to the cylinder. We were looking for the source then an operator crawled up to check expansion tank levels and an engine two down from the one we were working on had lost all of its coolant. I am supposing that the head pressue was enough to push past a bad seat or maybe even a improperly installed valve. Still trying to determine which valve is leaking.