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 Denali Power Brake Units May Leak Fluid From Hydro-Boost Unit
GMC Yukon Denali Problem
Average Mileage: 140,666 mi (94,000 mi - 178,000 mi)
Visitor, 2003 GMC Yukon Denali
leak at booster from power steering
Visitor, 2003 GMC Yukon Denali, 150,000 mi
hydroboost failure was triggering the Service Immediately lights and applying the brakes. replaced hydroboost for around $900 and it fixed it.
Visitor, 2005 GMC Yukon Denali
Power steering pump went out and then hydro boost starting leaking
Visitor, 2006 GMC Yukon Denali, 178,000 mi
Once the breech occurred, almost impossible to make right turns and braking is EXTREMELY difficult.