I am new here and have done a search with no luck. A few days ago that when I turn I am getting a hissing noise at the drivers side firewall when I turn the steering wheel but it goes away when I step in the brake. Has anyone ever had this issue and if so what was it that was causing it? I have checked the powersteering and brake fluid and they are both fine. Any suggestions would be great. Thanks

 

Do a PS Flush.

 

Its the hydroboost. Ford designed these trucks so that the power steering fluid goes through the hydroboost before it gets to the steering box. Mine was doing the same thing but I was still able to drive it for about a month then I finally got around to replacing it. I think theres a diaphram or something like that inside that wears out or gets a pinhole leak in it and thats what causes the noise. I put a new one from Rockauto and it's been fine ever since.