Cooling system overheating problem.

My 07 SD has just developed a problem with the cooling system, which is causing it to overheat. this seems to pressurize the system which causes the reservoir tank to dump water from the drain pipe at the front of the engine. It did this for the first time ever yesterday. Filled it back up when I got home (forgot to lift it 50cm) but it only took half a pint. Did a 40 mile run last night and it seemed cured. Now this morning, 25 miles from home, it does it again. When I let it cool down and slowly nursed it home, the temp went back to normal and after a few miles I was able to ride it all the way home, keeping low revs and down to 50mph. When I got home it seemed ok but the res tank was empty again.

Couldn't happen at a worse time, I go off touring in Germany in 4 days.

The rad cap is good, no probs there. The rubber cooling pipes were changed last winter and are definitely not leaking, and as I say, it's shown no signs of this problem before.

What can be causing this problem?

Airlock in the engine block?

Thermostat,,,,,,

I say thermostat. But with a small t. I may also pick a simple thing as a burp in the system. Does the fan motor kick on when the gauge shoots up?
does cap have any residue around it?
This sounds just like a problem I have had and the truth was that the sending unit to my meter was bunk or tired and that the temp never was what the gauge was indicating.
If the water usage stabalizes and the fan does not kick in when said red flashing occurs then ride on.
Once electricity could be a cause of alarm on the SD all bets are off that the true culprit isnt what you think it is and bets are on that it's the BGC or area near there.

Thanks for all your suggestions guys. Sorry for not replying, I've not been getting a prompt that I've had a reply; don't know why.

Anyway, I've ended up starting another thread because the problem now seems to be that the cooling fan isn't kicking in when the engine overheats. I've tested fan with a seperate battery wired in at the last connection point on the loom and it's fine. So it seems that it isn't getting power. maybe the coolant temp sensor isn't working or something welse in the electronics has failed.

Thing is, I haven't a clue where to start checking this out. How do I check the sensor? What other things need to be checked and how? And time is against me here because I'm booked on a ferry to Germany in 3 days on the SD. Fast help from guys who know this bike's systems would be really good.

Cheers

Where are you? I have a spare temp sensor but firstly check wiring continuity between fan and sensor.