I hate to digress, but there are a couple of things I would like to clarify now that I read your original message more carefully.
#1)How many miles does the car have?
#2)How long have you had the car?
#3)How many miles did the car have when you bought it?
OK, enough with the 50 q's, the point is that if you are a new owner, there is a chance that one of the prior owners replaced the front calipers and wheel cyliners a while back while the rear ones could have been the originals. You would know this by checking former records or, or of course, if your are the original owner.
If that is the case, in your mechanic's defense, it is possible that after he put new pads on the back with the old cylinders, the cylinders overpressurized and ruptured (happened to me on a classic american car). It would have not done anything to the front because they were newer.
It is very possible that this is the case because the front breaks wear out twice as fast as the rear, hence replacing the fronts in the past is not all that out of the question if your car has more than 100k miles.
There is also a distant possibility that he put on the wrong pads. The distance between the pad and the rotor have to be accurate to avoid overpressurization. Slapping generic pads on old brake equipment could be a disaster. It is also possible that they did not turn them down properly as well.
I first read your post in the middle of the night, but after thinking about it, there are a lot of factors that need to be considered and researched to ascertain who is as fault.
My best take is that you are a new owner of the car (less than 1 year) and your mechanic tried to save you money by taking a conservative inexpensive route, and due to the age of the componenets, cause addition malfunction. Unfortunately, this is common for new owners of used cars. Don't get too discouraged, only a small price to pay in the long run to enjoy the finest of automobiles.
....That'll hold true, if I knew anything.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)