Have you tried to loosen the bleeding nipple? I always do that when I try to push the piston back in. I put a good spanner between the worn pad and the rotor, and turn it with another spanner slowly. It will push the piston back. Then tighten the bleeding nipple again. Top up the fluid reservoir if necessary. It always works for me. Give it a try, nothing to lose.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.