If you had fluid leaking from the front calipers, you had only rear brakes! You may be quite surprised at how much better they work after you rebuild them.
Calipers are easy -- you can "lift" the piston out with a pair of screwdrivers to remove them. Make sure you coat the new seal with brake fluid before you put it in, and make sure you have the piston aligned properly before you push it back in, it's a pain to rotate it once it is in place. There are actually one three parts on each side -- seal (square, goes inside), dust boot, and heat shield. The heat shield will help you get the piston in correctly, it won't fit if the piston is rotated wrong!
Use a sharpened popsicle stick or plastic something to pry out the old seal - if you use a steel implement, you will scratch the bore and cause trouble.
Flush out all the old brake fluid and I think you will find the master cylinder is fine -- only if you have a brake pedal that drops to the floor and doesn't use brake fluid would I suspect the master cylinder.
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!