Remove the two screws, remove cap do not turn it upside down & place on kitchen paper, place a rag around reservoir & remove rubber seal place on paper then wipe around top edge of res. Check brake fluid level is not overfilled.
Clean rubber seal check condition & clean cap of all fluid,rebuild but don't lean on the screws to much they only need to be snug,wipe around cap then keep an eye on it, you may not need a new seal.