"Alfred K." wrote in message
...
Consider an extreme situation: a fully opened faucet and a completely
closed pop-up bathroom sink drain. Should a properly installed overflow
opening actually prevent the sink overflowing? Or it is supposed to just
help the main [open] drain by lowering the about to overflow water level
down faster?


Generally the overflow enters the main drain just below where the drain
pop-up closes. Therefore if the pop-up is open and the drain is not
draining to the point the water backs up to the overflow, you are probably
out of luck since the blockage is probably beyond (down stream) from where
the overflow would connect.