Power brake systems using "hydro-boost" may leak power steering fluid from the hydro-boost unit located behind the brake master cylinder. Replacement of the hydro-boost unit is the common repair to correct this concern.Google+
Car Problem Reports
GMC Yukon Power Brake Units May Leak Fluid From Hydro-Boost Unit
GMC Yukon Problem
Visitor, 2003 GMC Yukon, 125,000 mi
Leaks power steering fluid between the hydro-boost unit and master cylinder. Have not fixed yet, just add power steering fluid when needed.