I had to replace a cast iron T with an abs wye in my crawlspace.

I used the 4" fernco couplings and cranked the bands tight, but a very slow drip is still present. The cast iron is rusted and difficult to get a tight seal. What can I do to ensure a seal that won't leak?

Clean cast iron best you can with wire brush. Put film of silicone inside Fernco and reinstall.