What year is the CRF450R? I have friends who all had a 02-04 450 at some time, and all of them had leaky water pumps. I all but one case, the water side seal was in backwards at the factory (3 in all). On the last one, the seals were just worn out and the water pump impeller shaft had a small groove it in. Another issue is old seals and higher pressure radiator caps. There is some talk about an impeller issue that affected the 02-04 450s and was fixed in the 05 (I think). Something about a cavitation issue and the newer ones had a small hole drilled in them to help with it. If the bike is not over heating and the coolant loss is ONLY at the leaky water pump, chances are it is worn seals and at worse case, worn impeller shaft or bearings. Not a big deal, all are mounted in the right crankcase cover (clutch cover) and can be done with the engine in the frame.