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What I've done is turkey baster the res. and disconnect the return line from the res and run the line down under the car to a bucket. Cap the return port on the res. Lift the front end and turn the steering wheel from lock to lock until empty. Add fluid and keep turning the wheels, run a bottle of fluid thru the system. Reconnect the lines and refill the res. tank. Do all this with the engine OFF.
Once you refill it, turn the wheel lock to lock until no more fluid needs to be added. Now start the engine and continue to turn wheels lock to lock and top off the fluid. Lower front end and you're done. You may hear some air in the system but it will self bleed quickly enough.
This will purge just about all the fluid. The GM fluid is $7. I'm not sold on Valvoline PS fluid. The cars I've added it to have all made noise in the rack. I drained and refilled with stock fluid and noise went away. After it happend twice I felt it was no coincidence. Some guys like it though.