To prevent liquid bacKflow into the air distribution piping, a rubber check valve can be provided.
Slang definitions & phrases for rubber check
A check that cannot be cashed because not enough money is on deposit
[1927+; because it ''bounces'']
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with rubber check
A check drawn on an account without the funds to pay it, as in He's been handing out rubber checks right and left, but the police have caught up with him. The rubber alludes to the fact that, like rubber, the check “bounces,” in this case back from the bank.
[ ; c. 1920