There is no good way to flush a PS system. Even the flush equipment that you buy simply sucks the reservoir down and just refills it numerous times. The easiest way may be to remove the return line of the reservoir and point it into a drain bucket. Plug the fitting on the reservoir. Now fill the reservoir with fresh fluid and run the engine momentarily to run it through. It will only take seconds to empty the reservoir and you don't want to run it empty so you have to keep repeating this.