Rear calipers are stuck. Probably time for a rebuild. Alternate failure is a bad rear brake line or two -- if you did the fronts, replace the rears, too. Flush out with DOT 4 fluid, it's better.

When reversing, the rear brakes are going to do most of the work, the effect is worse on a grade. Stuck rear brakes will make the front tires slide on gravel.

Check the rear rotors for rust on the friction surface -- it there is any, they aren't working.

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!