Wipe the water pump clean and see if the leak is from the shaft/seal area. I see a lot of them leak there. I rebuilt mine just as it was made at the factory, and it never leaked a drop in over 10 years. The secret was to use grease all around the new lead packing rings, then snug the brass nut, run the engine, then snug the nut again. It doesn't need to be tight, just snug.
When I first bought my 28, it had a damp area near the upper right on the front of the radiator core. I used some radiator sealer, and it's been dry ever since. Even the vinegar treatment didn't open any new leaks.
As John mentioned, be sure that lower steel pipe is in good condition. Mine had some rust pits on the inside of the pipe, and one of the pits opened up while I was 50 miles from home. It happened as I was climbing a long fairly steep hill, and I didn't know about it until I heard a strange noise at the top of the hill. That was quite an overheat, and it took 2 gallons to refill and make it home.
About 10 miles down the road I stopped at a gas station food store to refill my 2 gallon water jugs. They wouldn't give me any water, but said I could buy their expensive distilled water. I left and made it home OK without their overpriced water. BTW I had some electrical tape under the seat to patch the hole in the pipe.
