G0RSQ

David Aiketgate wrote:

I would eliminate the master cylinder first. There is a valve within it that is supposed to allow brake fluid back into the master after you release the brake pedal. That could have failed.

When the brake peddle is released the piston in the master cylinder should move to a position that opens up the brake lines to the fluid resevoir.
This is to both release excess pressure in the fluid due to brake heat, and to allow fluid to replenish the slave cylinders as the pads wear, and pistons move further out.
Fault does sound very much like this is not happening, either through a faulty return spring in the master cylinder, sticking brake peddle not fully returning, or a damaged seal in the master cylinder blocking the fluid path from resevoir to the brake lines.