Radiator reservoir tank completely empty

[Danger]

I just returned from a 3,000 mile trip. That’s the interval for oil/filter change for me. When I removed the left side lower cowl the reservoir tank was completely empty.
It never overheated. To the best of my knowledge the hot fluid from the radiator expands and goes into the reservoir tank. As it cools it returns to the radiator. Obviously the bike had gone through many hot/cool cycles so I suppose my knowledge is wrong or something happened to upset that cycle recently.
What are your thoughts.

PS Here’s the bike. I tested my latest foot pegs. They worked fine for me.

 

[dwalby]

On the 1100, which I assume is similar to the 1300, usually the opposite happens. The line from the radiator to the overflow tank develops a small crack, which still allows the coolant to flow from the radiator into the overflow tank while the engine is running, but after its shut off the crack prevents a good vacuum from sucking the coolant back into the radiator, and the tank eventually overflows.

One possibility is you have a leak in the overflow tank, and it slowly loses coolant over time. Or, your radiator somehow got very low and it sucked all of the coolant from the overflow tank.

Check the coolant level at the radiator cap first, that's the important thing, and top it off if its low. Then fill the overflow tank to its normal level and keep a close eye on it. Also keep an eye on your coolant system for signs of leaks. Do a few local runs and see what happens after each ride.

 

[Igofar]

Look at the right side of the engine block near the oil inspection window.
You’re probably going to find a green stain on the side and front of the engine from a coolant leak that you didn’t know about.
These bikes never leave signs on the ground, as the coolant steams off when it hits the hot engine block.

 

[dduelin]

Every ST1300 eventually weeps, seeps, or leaks coolant. In a pressurized system the coolant tank accepts or returns excess coolant in the normal heat/cool cycles, just as you described but as it loses coolant from weeps, seeps, and leaks the tank level drops. When the tank is empty there is still sufficient coolant but continued operation would have eventually shown an overheat situation.

 

[RaYzerman]

What dwalby says...... plus, I had an ST13 which had a hole in the reservoir caused by melting from heat...... replaced reservoir and put some foil tape on it as kind of a heat shield.....

 

[Danger]

What I should have included in my original post is the fact that the radiator was completely full. After refilling the overflow tank I checked the radiator and it was full.

A slow leak in the overflow tank would seem to be the logical conclusion.

Thank you guys for your input. Very helpful.