I discovered the same problem (or at least symptom) on my '95 E320. It turns out I bought the wrong brake pads. They are shifting in the caliper. When I backup and then apply the brakes clunk. When I then go forward and on first application of the brakes, clunk.
I have to remove the pads and replace them with the proper ones. I bought them through someone on this site. They were "supposedly the right ones". I'll never do that again.
Take your car back to where you got your work done and tell them you have this problem, and you are suspecting the brake pads are rocking in the calipers.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)