When servicing the power steering pump, inspect the power steering fluid. Look for foaming, milky power steering fluid, low fluid level and leaks in the hoses and lines.
If the fluid has a foamy or milky appearance, it could be caused by air in the fluid and loss of fluid due to internal pump leakage causing overflow. Extremely low temperatures will cause air bubbles in the system if the fluid level is low. If the fluid level is correct and the pump still foams, look for air entry (cracked housing, etc.).
Your W-Body vehicle may be equipped with the MAGNASTEER® system, a speed-dependent power steering system. The system provides power assist at varying levels depending on need. A bi-directional magnetic rotary actuator in the steering gear adjusts the amount of torque (driver effort) necessary to turn the steering wheel. The MAGNASTEER® system is controlled through software in the Antilock Brake System (ABS) electronic control module or the Electronic Brake Traction Control Module (EBTCM) and varies the amount of torque by adjusting the current flow through the actuator. The amount of adjustment will be directly related to vehicle speed. The controller receives speed-related input signals from the vehicle speed sensor in the transaxle.
GM recommends their Power Steering Fluid 1052884 (pint) or 1050017 (quart) or equivalent. A special cold weather fluid is available under part number 1234586 (pint) and 12345867 (quart). Please note that GM specifies that the system should be flushed and bled prior to switching fluid types.
Power steering fluid level is indicated by marks on the fluid level indicator on the fluid reservoir cap. If the fluid is warmed up (about 150°, hot to the touch), the fluid level should be between the HOT and COLD marks. If the fluid is cool (about 70°), the fluid level should be between the ADD and COLD marks.