I had this pulsating pedal in my brakes once. It turned out to be either a
bent rear wheel bearing, bent bearing housing, or bent wheel drum. I
replaced all, so I dont really know exactly where the problem was.This
problem occured in my vanagon because I had it up on cement blocks, and a
block crushed. Lucky for me, I was in the bathroom when this happened.

I say this because a mechanic can damage the rear wheel area , like mine.
Also, I hear that a warped drum is 95 percent of the times caused by
improper tightening of the lug  nuts. Try loosening each nut and
retightening them.Your bearings and housing are most likely still good.If
this dosent fix it up, maybe try turning the drums to  get them round.I
dont know how this problem began for you. I suspect the lug nuts are the
issue. Good luck.