Active ABS will cause a pulsing in the brake pedal that you can easily feel.
And since you don't mention an ABS light on the dash I wouldn't think the ABS system is the problem.
Since rear brakes are often over looked I would start there.
Look on the inside of each rear wheel for signs of leaking brake fluid.
Set and release emergency brake several times, if you don't use the e-brake every day, then start using it, it keeps the rear brakes in correct adjustment to drums.