Details have been posted many times. Expect to replace the one caliper with the badly stuck piston as either the bore or the piston (or both) have probably been scored badly by forcing it back in.

My advise, normally, is that if they are that hard to move DON'T force them in, stop and get the rebuild kit since you are going to have to rebuild them anyway, and it's much easier to get the piston out as it.

DO NOT use compressed air to get that piston out for the rebuild, if it does come out, it will blow out at about 100 mph and could easily kill you if it hits you. Clamp the other piston in and pump it out with the brake pedal -- much safer, won't hurt anything.

Do not use abrasives on the bore below the piston seal (you will need to remove the corrosion above the seal, for sure) or at all on the piston -- the piston is chrome plated, and if you scratch the plating, it will both leak and probably peel the chrome off eventually. Make sure you get ALL the grit and dirt out of the bore, else the piston will stick again.

If there is corrosion in the bore below the seal, or on the chrome plating of the piston, get another caliper.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!