Both rear wheels, or just one of them?
If just one wheel locks up then suspect the rubber brake hose which connects the metal brake lines on the body to the caliper mounted on the wheel. I always change all four rubber brake lines on cars this old - the brakes are the primary safety system on the car.
If both brakes lock up then it's going to be the master cylinder. Rust in the bore of the master cylinder causes the piston which actuates the rear brakes to jam in place, locking on the rear brakes. This is a known failure mode of the master cylinders used on Benzs in that time period - I've experienced it first hand.
Has the brake fluid been changed per the maintenance schedule? Changing the fluid prevents the rust which causes this problem. I bet the fluid is nasty and full of absorbed (or is it adsorbed, I can never remember the difference...) water.
Anyways, either fix is relatively easy and inexpensive. Just don't let a mechanic sell you new calipers, rotors, etc if you don't need them.