This could be caused by low brake fluid.It would be a good time to change the brake fluid as brake fluid draws moisture over time.You don't have to drain and refill,you can start by suctioning out most of the old fluid from the reservior and refill with new fluid.Next go to the right rear and loosen the bleed valve and let the old fluid drain out untill the new fluid that you added starts coming out the valve.Do the others in turn doing the next furthest one from the master cylinder untill they run out clear.Remember to keep the master cylinder full or you will get air into the system and have to bleed the air out.