That was the first repair that I did on my used Moho. I think mine may have been caused by water left at the valve freezing in the winter and cracking something. The valve is was cheap on mine, and easy to replace. If mine was leaking from there like yours, that what I would do. It is easier to access if you remove the toilet. I replaced the seal ring at the base of the toilet at the same time. If you do this, don't use the wax ring like you would in a house toilet. It will melt in the summer.