Testing New Construction for Plumbing Leak
Close all valves and seal all but one opening in the home's plumbing system.
Apply a steady 10 pounds of air pressure to the single opening.
Stop and retest. If there is a drop in air pressure, you have determined that there is a leak in the new construction plumbing.
Locating a Plumbing Leak
Turn off all valves, and seal all but one opening in the area where you will be working. For example, if you are detecting a leak in the kitchen sink area and have a double sink, you must seal off one of the sink drains. Use a bunched-up plastic grocery bag and a heavy item on top of it to seal off the open drain. It does not matter which drain you choose, as long as one is open and the other is sealed.
Plug the hose of a reversible shop vac into the outtake hole so that it becomes a blower instead of a suction tool.
Have a second person hold the blower over the opening in the section of plumbing that you are checking for a leak. If you do not have a shop vac, rig up a way for the second person to blow air down the open section of plumbing. You can use a piece of hose or rubber tubing and a bunch of plastic bags to form a tight fitting on the open end of pipe and then have your friend blow air through the tubing. A small amount of air is sufficient for this test.
Run a piece of tissue paper along the entire section of plumbing that you are checking for a leak. The tissue paper will easily slide behind even a tight section of pipe. You may need to hold your breath during this test, as tissue paper is very sensitive to air movement. Keep your eye on the tissue paper. When it flutters, you have found your leak.