Hi,
the worst brake mal behavior I had once was related to the rubber pipe which was puffed inside and did not let the brake fluid to flow equally in all wheels, but delayed to the puffed one depending on how hard you hit the pedal.
Nothing was to be seen from the outside tough, renew the rear rubber pipes as you are working there, those are almost 10 years old. From the car behavior one could say its the rear left one?
good luck
Tapsa