I was dog sitting for my parent's 10 year old Golden/Lab mix, Scout. She started the leaking while she was here. I thought it was an infection and my vet treated her with antibiotics. It didn't help, so he put her on a female hormone (sorry I can't remember the name). She takes it on Sunday and Wednesday, and is much improved. The minute my parents forget to give her the medication, the problem starts again.
My vet said that it is age related, and as someone mentioned before, very common in females that have been neutered.
Good luck with the problem.