1973 Ford F-250 power steering pump

I was driving on the fwy when I smelled fluid burning. I found my power steering fluid was over flowing out the fill tube. And now the pump was making a grinding noise. I went to the nearest auto parts and and refilled with powersteering fluid. I drove my truck about 10 or 15 miles and rechecked. The noise went away and the fluid was not over flowing again. The next morning on my way to work I was on the fwy for about 20 miles and started to smell fluid again. I found the same thing again? Fluid comming out the fill tube. So after work I removed the pump, installed a new pump and changed both the high pressure side and the return side hoses. Flushed the system and drove the truck around town an checked. Everything seemed ok. The next day I was again on the fwy for about 25 miles and I started to smell fluid again! The fluid was overflowing out the fill tube. It seems to me that it only happens when I have driven at a higher rate of speed for a longer distance? I'm at my witts end. My truck is old, but I completly restored it 7 years ago and people are always telling me how great it looks. I know a little about working on cars, but this is pissing me off!