Plugged overflow bottle
 

I just changed coolant and found that my overflow bottle wouldn't drain thru the hose. I used my mityvac to pressurize the hose and it took 30lb of pressure to clear the blockage.The sediment would immediatley block again,so I held my compressor to the hose while flushing the bottle with a garden hose so the sediment would float out. It's now functioning properly,as it was the radiator was a sealed unit unable to overflow under pressure. So the next time your doing service you may want to check yours.

Did away with mine all together on one of my other bikes, as it cracked and a replacement was just too dear for logic. I check the rad level every service and never need to add, so no problems there. As it would appear yours was blocked, it was not functioning anyway.