You are correct: the majority of the braking power comes from the front brakes--about 70-80%. That's why the front brakes pads are much larger than the rear, I suppose. If you want to see how the back brakes are working, take a thick black, water-based marker and scribble all over the rear rotors. Check it after running some errands to see if some of the markings have been erased. If you see no eraser marks, re-grease the caliper slides and assure the pads are correctly seated.