Bulling the Barrel
is pouring water into a rum cask, when it is nearly empty, to prevent its leaking. The water, which gets impregnated with the spirit and is very intoxicating, is called bull.
Seamen talk of bulling the teapot (making a second brew), bulling the coffec, etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894