I was having this problem since replacing the rad on my Vee and recently running it in hot weather.
The solution was to fill the overflow tank to between the lines, run the bike 'til warm, then lean the bike WAY over both ways with the engine running.
The shop manual indicated to do this procedure, but I blew it off and as a result, had trapped air in the engine somewhere. Apparently, leaning the bike over allows the air to escape back into the overflow bottle.
I've run the engine to 3 bars a few times since 'burping' it and it hasn't overflowed yet.
It worked for me, anyway...
Good luck.
The solution was to fill the overflow tank to between the lines, run the bike 'til warm, then lean the bike WAY over both ways with the engine running.
The shop manual indicated to do this procedure, but I blew it off and as a result, had trapped air in the engine somewhere. Apparently, leaning the bike over allows the air to escape back into the overflow bottle.
I've run the engine to 3 bars a few times since 'burping' it and it hasn't overflowed yet.
It worked for me, anyway...
Good luck.