Watch when it drips. A leak while the sink sits idle is on the supply side, meaning a supply line or stop valve. A leak only while water drains is on the drain side, meaning the trap or its connections. Dry every joint with a paper towel and see which one wets first.
How do you tell which one it is?
Wipe the pipes and fittings dry, then put a dry paper towel under each joint.
- Supply side. Pressurized pipes and stops leak around the clock, even when nobody uses the sink. Look at the stop valve stem, the nut where the supply line connects, and the line itself.
- Drain side. The trap, its slip nuts and the tailpiece leak only while water is draining. Fill the sink basin and release it; a wet joint under the trap shows it.
- The fixture itself. A faucet or sink strainer can drip into the cabinet and look like piping.
EPA's commercial guidance calls for examining fixtures and fittings for visible water, and a daily walk through kitchens and restrooms to catch drips early. The simple version of that is a quick look under the sinks at close.
What is the usual fix?
Supply and stop leaks are parts jobs. BrassCraft's instructions for its supply stops warn that overtightening can cause the product to crack and fail over time, and say to inspect the stops annually and replace them if damaged or corroded. A stop that keeps weeping after a snug nut is a replacement, and the water has to be shut off upstream first. Angle stop failures are covered separately.
Trap leaks are often a loose or worn slip nut, a cracked trap or a bad washer. A trap that is leaking because of a partial blockage needs the drain cleared too, or it will leak again. The residential version of this diagnosis walks the same steps.
Why does it matter more in a food business?
Maricopa County's food code incorporates the FDA 2017 Food Code by reference, and Section 5-205.15 of that code requires a plumbing system to be repaired according to law and maintained in good repair. A cabinet leak is also a floor and cabinet hazard in a kitchen that stays busy. If the sink must be taken out of service while it is repaired, plan for hand-washing coverage first.