Similar problems I have seen personally were;
1. Flexible line broken on inside allowed fluid to make a bulge on the outside portion of the line. Held the one cylinder longer than the other. Bulge would be gone before I got out to look at it. Found it when someone helped push on the pedal while I looked. When pushing pedal and holding firm, pedal would creep down similar to air in lines.
2. One of the junction blocks in the line was clogged. Piece of junk would clogg when pressurized, but release in the reverse.
3. wheel cylinder corroded just enough to hang, but still release.
4. brake pads oily and cleaned. Need to replace pads at this point.
