My truck started hissing about 1 1/2 weeks ago. This is what I did to fix it. I first flushed the system just to see if that would fix it. It helped a little, but got worse after a little driving. Then I checked the lines, seals, and the power steering cap. They were all good. So I replaced the hydroboost, refilled the powersteering box with ATF+4 fluid, and bled the system. Now it steers and brakes like a new truck. The part was $165 at NAPA plus the cost of new fluid. It took me about an hour to swap them out in my driveway. I don't know how much the dealer would have charged, but I'm guessing at $150. It was a little messy, but I have a pressure washer, which cleaned everything up like new. I hope this helps someone.

 

Good deal man!