Another analogy like the sink is the garden hose.   I will sometimes tell
clients think of a garden hose that is kinked (the breast prior to feed )
 release the hose (your letdown) the water comes out forceful and will
continue to give water until someone turns it  off (baby taken of the breast)
 Also, I too try to avoid the term "empty breast" I say "milking the breast"
adequately and/or correctly.
Annette RN,BSN