The vacuum booster isn't helping you brake the car; it's not working so
you now have to overcome 1 BAR pressure against your brake pedal
1. Check that there's vacuum at the hose connection to the booster with
the engine at idle.
If not, that's the problem and you need to find why not.
Hint: Could be a collapsed vacuum hose to the engine's intake manifold.
2. If vacuum is present in the booster end of the vacuum hose then the
booster is suspect and may need to be replaced. Given the car's age you
may need to go MBUSA's "Classic Parts" Department for a replacement or,
hopefully, a repair kit.
Someone may know how to rebuild the booster, for these old cars were
designed to be repaired rather than have their parts replaced.