As she hasn't been spayed, the likely cause is hormonal disturbance possibly. How many weeks is it since she was in heat? Nipples leaking fluid CAN be a symptom of "False Pregnancy" (which is caused by hormone swings)
They can also be a symptom of breast tumors. Even a very tiny benign cyst in the breast can cause a leaking nipple.And sometimes, when gently squeezed, the nipple fluid can be brown-ish. I know because my dog has a couple of benign cysts, and they used to cause that symptom. However, after she was spayed, that stopped. (Although the cysts remained. I decided, as they are benign, and seem to cause her no trouble, not to have them removed and put her through surgery for them.)
Mastitis is also a possibility. But with Mastitis, you would probably see redness, swelling, and a "hard engorged" feel to the breast tissue.
But that doesn't mean it's definitely not infection of some kind. It might be.
If I were you, I would take her to be seen by the vet.
If you don't intend to breed from her, having her spayed may be a good option. Females with regular hormone-swings are in greater danger of developing Pyometra. And Pyometra is very very dangerous for a dog to get! It can kill dogs within hours, or leave them with kidney failure.
Ginger gave you excellent advice. To add one thing to what she said, the older she gets, the lower her odds against avoiding pyometra. It is very common for unspayed females to develop it, especially later in life. It would be a wise move to have her examined by a vet to determine the cause of the leakage, and then have her spayed to totally eliminate the possibility of her developing pyometra. Eight is about the oldest that I would consider breeding a dog anyway, so it's not like you are going to shorten her breeding life if you have her spayed. By this age she should be able to relax and just enjoy life. At eight years of age, a King Charles Spaniel is the equivalent of a 48 year old woman, not old by any means, but old enough to not want to deal with the rigors of a pregnancy and the turmoil of raising kids.