The rear hose may look fine but it could be swollen on the inside. This happened to my dad with his old immaculate MG Midget. Failed the MOT on dragging rear brakes. Couldn't work it out at first as the hose looked like new (original part). He just bit the bullet and bought a new hose and fitted it (he already replaced the front hoses). Bled the brakes, no more issues and much better brakes, too. Cut the old pipe to have a look and low-and-behold it was swollen closed on the inside, allowing pressurised fluid through, but no return.
Front ones seem prone. Rubber collapses on the inside so t acts as a one way valve. Press the pedal and the fluid operates the brake. Release the pedal and the fluid is trapped by the hose and won't release the brake.
On the fronts, you can usually push through it and force the brakes off.
I would imagine getting the rears to release would be more difficult.