What rights did servants in medieval time have?

That'd depend on the country and the status of the servant and the employer.

Things ran a pretty wide spectrum, from outright slavery on up.

They had the right to NOT wear masks

The Middle Ages covered over a thousand years and many countries.

Do you suppose their rights changed over that time or varied from country to country?

William Wallace had a Sterling Bridge 

depends because in denmark the "servants" had no rights lol

There were no servants.  There were no serfs.  There were no feemen.  There were no rights.  There were killers.  There were killed.  Killers reproduced.  Killed did not reproduce.  Our ancestors were killers.

None. They lived, worked, and died at the whims of their lords. 

right to work as long as the payed there taxes , thats all .Kind  of like new !!! troll .

the Right to serve their masters

Medieval Era trends was to keep maids for each house as they live in larger house made with think walls in ancient Europe.