What do you mean the fluid in the system? The reservoir is cycled through they system. You can drain it all out, there is a Video on that. bythabay's thread has it.

 

The process for draining the entire system is a little more involved than the reservoir drain and fill method. True, there be be some sediment in the lower part of the system that is not circulated out by the reservoir method, but probably not much as the system is closed (sealed).