Open up the junction box and start with a few drops of Dawn dish soap down the conduit. Do this for a few days. Should be good to go then.

As for pulling the new wiring up, here's what I do;
Remove the old light from its housing and set it on the deck. Cut the wiring at the light's base. Take the jbox end of the new light's wiring and place it end to end with the old wiring. Use gorilla tape to secure them together (both around and length-wise). Pull firmly and slowly. Should be good. This method hasn't failed me yet.
Remember you only want enough wiring that you can remove the light housing and place it on the deck. Any excess will make it a nightmare to get the light back in the niche.
Hope that makes sense, not sure I explained it well.