I can knock a block out of a railcraft tank, replace it, and the liquid inside will be magically restored. However I just expanded my 5x5 tank to a 7x7 tank by knocking out a corner of it and the liquid did not come back.
Under what conditions will the liquid inside a tank be kept? When will they not?
Note that my experience seems to contradict the notice on the wiki:
If you have an accident with your tank and all your liquid appears to disappear don't worry as long as the bottom center block is still intact, replace the broken block and your liquid should reappear.