Maybe I'm just stupid ? I'm sure lots of you may agree. But I figured something out a decade or so about fire nozzles. I had a line running along the inside of the frame rail to a Y behind the radiator of my Street Roadster. (Which reminds me of a discussion of a venerable Road Runner racer with an inspector one day long ago. "You mean my nozzles are in the wrong place? Just tell me where the fire's gonna be and I'll move them over there.) Any who, I branched them off with lines running up on either side of the "radiator" and pointed back about valve cover gasket level. Years later, getting ready for Bonneville, I was blowing out the lines (a good practice once a year with the nozzles removed) when one of them had an obstruction. I was using brake lines and one was rusted internally. How did that happen? By hosing off the engine after every meet, some water apparently went in the nozzle and stayed.
Easy fix ? put the nozzles on the down side of a supply line and any water will drain out.