[wslade2]

By sump tank are you meaning the one in the bottom of the boom between the Main tanks and Aux tank? That is reachable by removal of a curved plate on the bottom of the boom secured with screws. It will have a scupper drain tube sticking out of it and a water drain valve hole. I have accessed it to service it (but not to remove it). I attached drawing from parts manual to clarify tank nomenclature. Sump tank is #56. As mentioned in the service manual, you will also have to remove the aux tank (and upper inboard wing panels) to finish getting it out. So both bottom boom plate, upper aux wing plates and aux tank require removal to remove the sump.

Or with the language "inboard" are you just talking about the aux tank only? Either way looks like upper inner wing panel and aux tank coming off (as in pictures already supplied).

Those upper wing panel screws like to get frozen/corroded/rusted in place and may have to be drilled out/eased out/soaked in oil, etc. Be sure to support the wing. Upper panels provide tension support for the wing, gone when panels removed.

By the way, devils advocate here, sure it's the sump tank leaking? The mains have rubber hose connecting them that may be of questionable age and condition. Could these have deteriorated and allow fuel to run down the wing to low point which is sump area?

see attached