DFD536

So, here's the situation... I am driving down the highway, and as I get off, the brake pedal is considerable higher than normal (not physically, but there is none of the normal press before the brakes apply), as I continue to drive, the pressure builds, until the brakes are physically applied, and the truck will almost not move (it is literally like pressing down the brake pedal). Having never heard of this, I called my dealer, who was also at a loss, but said the brakes are powered by the power steering pump, and maybe low power steering fluid made an air bubble. I checked, psf was a little low, filled it, turned wheel many times, pumped brakes, still locked. As a last ditch effort, I opened the bleeder on the front brakes, and the fluid sprayed all the way to the rear of the truck! After this, everything seemed to work normal, but it is doing it again.. As I bleed it, it works for a short while, then the pressure builds behind the brake pedal until it will literally lock the brakes.
It seems to be the front brakes, not the rear... No recent brake work, a few months ago, had both rear calipers replaced, and system power bled...
I'm at a complete loss, dealer says I can bring it in, but will be difficult to diagnose since it has to drive 30 min or so before it will pressure and give me problems (I read that as it will cost ya!)..
I need ideas guys, anything you can think of may help..

thanks,
Dennis