Leak from expansion tank

Hi

Ive got a dt125r and ive been getting coolant leaking from the expansion tank ive ive finally found out the problem

Ive again drained the system and bled the system but what is happening is

I checked it this morning before i went to work and the rad was full and the expansion tank was between the levels

But tonight before i left work i checked again and about 3/4 of a pint had come from the rad and was in the expansion tank anyone got any ideas what the problem is

Thanks Mark