Geo Hahn said:
Another possibility (albeit one I have never experienced) is that the brake hose can fail internally causing a sort of check-valve effect and holding pressure. I believe this can occur even if the hose appears fine on the outside (inside either swells shut or a little flap develops).
Happened to me several years ago, although it was the rear brake hose. Looked OK on the outside, not new but noticeably weather checked or rotted. However it caused the rear brakes to drag enough to overheat in traffic. At one point they were literally smoking!
When I later cut the hose open, it appeared that the inner lining had torn somehow, and there was a little wad of material blocking the bore.
If cracking the bleed valve causes them to relax, then I'd first change the hoses, and then, if the problem persists, the residual pressure valve (on top of the 5-way union). (Actually, I just gutted my RPV years ago, because I didn't like the brakes dragging even lightly.)