Almost giving up on my VT...

I posted a thread about my VT being in the dealership for brake repair.
Well it has been sitting 17 days in the shop. The parts have come in and the calipers, pads and rims replaced. During test drive, the service guy said the brakes seem to work, but once in a while, especially trying to stop fast, the brake pedal would feel 'hard as a brick" to push. He test drove another VT from the lot and says the brakes on my VT really feel different.
He's not comfortable sending the car home with me, and asks for more time to diagnose the problem. I'm about done with the waiting, as my brand new car only spent 4 days at home, and more than 2 weeks under repair.

Any ideas what could be wrong with the brakes? Master cylinder, brake booster vacuum, anything?

Should I give up on this one, or should I give them more time to fix it? What can I do as far as giving up on this one and having Hyundai just put me in a new VT? Who can i bug to get this rolling? Should I call the dealership I bought the car at, and ask them to give me a new car?