My old radiator i did use water alot in it cause it had a leak and yes i drove around with a leaky radiator for over 5 years and just installed a new radiator last year which i filled with coolant and never with water cause didnt want it to rust up inside. My dad thinks its because of stop leak i put it over the years with my old radiator but nothing with the new one all i did was put in the new radiator and filled it with coolant and that was it till recently i put in water for first time and it was all brown looking.