This problem has been posted many times. Do search and you will get lots of info. However to start you off ,you need to switch to synthetic power steering fluid. One easy way to get the old fluid out is use a turkey baster. Suck out as much as you can and fill with synthetic . Drive the truck for a day ,and repeat the procedure two or three times. There are other methods you will find in the search. I have done this several years ago and no problems since. My truck has 494,000kms on original power steering pump . good luck with it.