Um, the parking brake has nothing to do with the rear pads. The parking brake is a separate set of shoes inside the hub of the rotor. I'm not sure how they explained the 50%. If this is the original set of brakes and the rears go before the fronts (fronts being what about 75% of the stopping resistance) I'd suspect a sticking caliper.
As for not feeling a difference with a fluid flush that's a good thing. By the time you can tell something is bad it's kind of too late. That's why they call it "preventative maintenance"