I just went through this on my DO, hopefully you won't find the cracked block that I did. Try to shine a flashlight up into the side covers. If it's leaking from the orings, you should see some green around the base of the sleeves. If not, the leak is somewhere else.
As for the broken cotter pin, try turning the nut slightly. The hole for the pin runs straight front to back and if the castle isn't perfectly lined up the pin won't slide. I broke a couple cotters before I figured this out.