Using a suction gun won't clean the system very well.

The proper and easy way to flush and change PS fluid is to remove the return line from the pump and lay it down in a drain pan. Remove the reservoir fluid and be prepared with about three quarts of Power Steering Fluid.

Have an assistant start the car and start turning the steering wheel back and forth lock to lock while you pour two quarts of fluid through the system. As soon as it starts blowing air after the second quart, signal assistant to kill engine.

Reconnect return line to pump and fill reservoir with fresh fluid. Have assistant start it up and watch fluid closely while the assistant turns the wheel lock to lock to bleed and take in fresh fluid. Add fluid until system is full.

This is the method covered on the ASE test. I have used this method on MB's and other cars with great success. It is quick and simple and cleans the system well.

Good luck,