I never thought to ever flush out my coolant reservoir. I did it two days ago and all I have to say is:

So much MUD came out. Its actually mineral deposits from using tap water with coolant.

H2O evaporates. The minerals don't so they remain as a deposit. This is the same deposit which turns your coolant to mud. Literally.

Want to see how much crap is in your cooling system? Disconnect and remove your battery.

Next, disconnect the hose running from the reservoir under the battery to the back of the radiator cap.

Remove the two retaining bolts and pull the reservoir out. Dump it into a bath tub. Wash it out and watch how much mud comes out. That mud is circulating in your motor.

 

I did that a while back....it really is kinda scary.

It took mine apart last year and rinsed it with a garden hose to get that thing cleaned out.

 

I bet this is half the reason some of the cars on this board run hot.

The passages are clogged.