Was this a Volvo dealer? Sounds like they put on non oem pads or the wrong pads for the car--the harder aftermarket pads make the car vibrate like a jet slowing down on the runway. The pulsating comes from the rotors being warped. What is this "something" they did? Dealers are not supposed to turn (sand) the rotors or sand the pads. I am surprised that your dealer did this?

Advice: (my opinion) Take the car to another Volvo dealer and ask them to look at your brakes, front and back, and tell you what they find. After this, explain to them what the other dealer did and the problems you have. See what they say. If they say something is done wrong, have them put it into writing and then go back to the original dealer and show them the paper and tell them to fix the problem. It sounds like they have already tried, but might have an inept mechanic. If they give you a hard time, contact a local tv station and explain your problem. Dealers don't like bad publicity. TV stations love this sort of consumer problem.

My feeling tells me that the dealer is trying to soak you for money. They fix the front brakes, then when you have problems, "Oh, let's do the rears". If that doesn't fix it, "Maybe you have other problems we can fix". JMO