My brakes on my 2006 Armada that I bought back in 2005, started locking down about two months ago. I took it in to the dealer and a few other shops, and like many other post's on this site, they said they could not find anything wrong. After two months, I fixed the issue, and when searching Google, this seems to be a common issue with other Nissan and Infiniti vehicles.
Here is what I did in this specific order:
1. Replaced both front brake lines and flushed old brake fluid from entire system and replaced with new fluid - because when I bled the lines, the brakes would release. There was a post that said the rubber brake lines inside the tire well could collapse internally, so the lines were replaced first.
2. Replace Master Cylinder - Still sticking, so I replaced this unit.
3. Replaced both calipers on the front only - Still sticking. This actually made it worse than it originally was, probably because they could slide easier, and probably because of the new brake lines.
4. Replaced Power Brake Booster Check Valve - This fixed the issue for around $6.00 and about 10 minutes of my time.
If you start having brake issues where they lock down, replace the Brake Booster Check Valve on the Brake Booster first. This would have saved me a lot of time and money too, but I have a whole new brake system up front on the plus side. The negative side, it cost me many hours of work, and around $700.00 in parts. I am convinced the $6.00 part would have fixed it the first time.