Coolant System

"When you opened the Expansion Tank/Radiator Cap, Did you find that the radiator was completely brimming full right to the Cap? " 

No - full almost to lip - took about 15mm to brink to lip.

This has perplexed me for a while - There are definitely no leaks (I have checked repeatedly) and yet I have toped up the Expansion tank with 10-15mm on may be 4 occasions (out of say 8 runs to cylinder head temps of about 115C).

My last 912 never needed topping up.

I have come up with two  possibilities;

There is still a small amount of trapped air somewhere in the system but even this should be driven out as the expanding coolant leaves the system & enters the overflow tank, to be replaced by coolant from the overflow, as the system cools - so unlikely.

The radiator cap is faulty and some air, rather than all coolant from the overflow, is being drawn back in to the system. 

Sean the quick and easy test is to replace the radiator cap. Triumph motorcycle part number T2108005. (Aus $25.45 3rd Feb 22)

It is more likely that the Expansion hose has a minor air leak at one of the fittings that allows the hose to drain between runs and the air in the hose is being drawn back into the system.
Be sure your overflow bottle is half full when cold.
If you have a clear Overflow hose you would see the hose full at the end of a flight and later see that it drained overnight.
If your overflow bottle is below the expansion tank, this hose will always have suction pressures in it.
Air will leak IN, but coolant will Never leak OUT.

Bill Hertzel wrote:

It is more likely that the Expansion hose has a minor air leak at one of the fittings that allows the hose to drain between runs and the air in the hose is being drawn back into the system.
Be sure your overflow bottle is half full when cold.
If you have a clear Overflow hose you would see the hose full at the end of a flight and later see that it drained overnight.
If your overflow bottle is below the expansion tank, this hose will always have suction pressures in it.
Air will leak IN, but coolant will Never leak OUT.

If  the Overflow hose has a small leak it must only be leaking under negative pressure, as there is definitely no coolant leak - still possible so will refit hose (get some clear).

Overflow 1/2 full .

My Overflow tank is below the Expansion tank - " this hose will always have suction pressures" from this statement, I understand that the suction pressure would be the very small head crated by the difference in height. However this will not "always" be the case - when the coolant temperature/pressure overcomes the radiator cap spring loading, coolant will enter the tube under positive pressure. The only time the Overflow hose will be under significant negative pressure, is when the engine/coolant cools down and the radiator cap valve allows coolant to return to the engine system.

 I am usurer of your related  statement "Air will leak IN, but coolant will Never leak OUT" how will the air leak in without a hole in the system?

The flow in this hose is extremely slow.
The cap does not suddenly pop open and dump a lot of coolant all at once.  It bleeds out slowly as needed to maintain the pressure. (~18psi)
As the engine heats up there might be 4 ounces or more of coolant dumped to the overflow over a matter of 5 minutes or more.
So maybe 1 ounce per minute.

On the return trip, the 4 ounces of coolant is drawn back in over the course of a few hours or so.

The hose will never see positive pressure, as the incoming coolant never flows faster than if powered by gravity only.
The negative/suction pressure will be extremely small and measured in inches of water-column created by the difference in the two hose ends but it will never go positive.
So Air might Leak in through a pinhole, but not a drop will leak out.