It may be possible to rebuild this caliper, but if the piston is stuck that fast you may be better off replacing the caliper. Check PartsShop and FastLane for prices.
Also, if you do replace the caliper be sure replace the opposing side caliper as well.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.