Make up an adapter for the water supply like the one in the picture or buy one and purge the water out of the system with the adapter and then just apply a regulated 30 pounds pressure to the system and listen for the leak. If a helper can not hold the air hose steady on the adapter while you listen make a permanent connection to supply the air.
If the leak is in the kitchen area all piping should be above the floor.
You can couple copper and pex directly together as long as the section of copper pipe is ROUND. Any straight section of copper should still be round.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles