Flushing the Hydraulic Booster System
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, 2004 Chevrolet Cab & Chassis Silverado, and 2003 GMC Sierra. However, it is still accessible from the selected car via links, so may be relevant.
Refer to Flushing the Power Steering System in Power Steering System.