Soak a paper towel with white vinegar. Lay the paper towels over the water stains.
Allow the vinegar-soaked paper towel to remain on the brass pipes for 10 minutes. Take off the paper towel and wipe the pipes with a dry soft cloth.
Cut a lemon into quarters. Remove the seeds.
Coat the flesh of one of the quarters with table salt. Rub the lemon flesh over the stained brass. Recoat with salt as necessary.
Wipe the brass with a dry soft cloth. Repeat rubbing with the salt and lemon until the stains are gone.