In our group, we see these problems all the time and it can even happen with units as new as yours. (Ours is not that new.) Just because parts are new does not make them good. We even have seen trouble with parts supplied by the original manufacturer label, but most are made in China and many have serious quality issues.
And yes, if you have not replaced the rubber brake lines, expect a problem with them.
What you are describing is exactly what a bad master cylinder will do. When you say it will pump up but not hold pedal position, I have a suggestion. Get a helper. With lid on, pump up and hold. Now one of you get out there with a good light and open the reservoir and look in. As the diagram shows, there are two holes at the rear location. Study that hole. If there is any evidence of fluid returning while there is pressure on the pedal, that is it. But, the master cylinder can also leak without actually returning fluid.
Good Luck Guy.
Matt
