leaking m/c reservoir?

[mini64]

I have the tandem master cylinder, newly sleeved and rebuilt, and with a new old stock reservoir I put on. Seems to be leaking at the rearward connection from the reservoir as evidenced by the rust, plus wet down on the fiberglass. Any ideas on how to cure this? I believe there is a seal on the plastic neck, but if I remember right it is a pain to pull the reservoir off without damaging it.