So after my last track day i had some coolant spraying in my face. Would only leak when on track.
i know it may sound stupid but is this nut supposed to hold under pressure? When i pressurize my cooling system its the only leak i can find.
Is your drain hose line working ? Perhaps you could fill the expansion tank to full level and let it warm in a controlled place, just to check that the exceeding liquid is dripping from the hose. That hose is located next to the rear brake pump(at least in B3800). Perhaps it is pressed or closed somehow.I Keep at min when cold. Man i checked everything i cant find the leak. I've even replaced all the hoses with samco hoses and clamps. The only thing i havent replaced is the hose from the reservoir to the top of the rad.
If you replaced the bolt and the washer from your picture, maybe the coolant is leaking from the upper black plug.