I agree with you that most likely the entire brake system should be bled front and back. I have seen in some cases that the rubber part of the brake line wears down and causes the small rubber portion to balloon just a little bit there for reducing they hydraulic pressure in the system. No ballooning effect with soft pressure on pedal(low psi in brake system). Hard pressure on pedal(higher psi in brake system) rubber portion in brake line balloons just a little enough to not allow max pressure builded up for caliper to push pad firmly enough on disk rotor. I've seen this before just not a lot.