buy a new one... replace the hydroboost, the power steering pump, and steering gear box if it's older, all at the same time, and the hoses too... once you tear into the system, and dislodge built up gunk, from personal experiance, others stuff is gonna need attention soon. I understand if you need to do just one for money reason, but if you can afford it, do the whole system, it's worth it. But deff the new hydroboost, and a new pump is worth it...'
Edit: after reading your sig, and seeing the low miles (105K) it might be less of an issue with your truck, not needing the complete system replaced...
Edit: after reading your sig, and seeing the low miles (105K) it might be less of an issue with your truck, not needing the complete system replaced...