Purney,

It sounds like you have it nailed. Soft plugs are that size, are in that vicinity...and they do get pin holes in them. Make 99% sure it isn't dribbling down from somewhere else before you spear the soft plug and pry it out. If accessible, a new one is relatively easy to install. Brass ones are available is you feel frisky. If you can't get a clear shot at it with a hammer to install the new one, I think they make rubber expansion types that work OK.

Maybe you could do the water pump and soft plug at the same time to save dumping the system twice. Good luck and make sure that water isn't coming from somewhere ABOVE the soft plug!

Stuart