Do you mean the valve on the side of the toilet, or the through hull seacock valve in the hull to let the lake water in. (unless yours is coming from the freshwater tank)
I have the same boat only a 2003 and recently had the same exact problem. I replaced the top valve gasket, its in the rectangle area where the pump handle is. Very cheap fix, took 5 minutes to replace and worked like a charm again. Link below has them pretty cheap. https://www.wholesalemarine.com/jabsco-top-valve-gasket-29042.html
Comments
https://www.wholesalemarine.com/jabsco-top-valve-gasket-29042.html
OBjuan
Go Steelers!!!