MB has a light return spring, however not all brands of cars use a spring so I suspect this is more of an anti rattle device than peddle return.

The power brake booster should return the peddle on it's own. With power brakes, even if the master cylinder is stuck the peddle will still return. With the engine off , press the peddle a few times to bleed the vacuum off. Does the peddle still stick?

If not, the booster has a bad control valve ( this usually can't be repaired in the field ). If it still sticks, disconnect the push rod from the peddle and see if the peddle is stiff to move, if not change the booster.