Typically it will be one side or the other getting stuck, which will cause the vehicle to pull left or right. This is usually the slide pins in the brake calipers causing this. In rarer cases the flexible brake lines deteriorate from the inside and can cause a check valve type issue so that the brake doesn't release on that side. Sounds like you've changed out all those parts.
The only time I have heard that both front brakes were dragging was due to a brake booster issue. In that issue the poster said he could see the rod between the brake pedal arm and booster get sucked in when starting the van. The link below may be a better place to ask your question to a wider audience.
FTE Link
