Thetford C3 flushing

Hello,
After many years of trying I managed to get the flush on my Thetford C3 toilet working, but now it won’t stop!
I replaced the blown fuse at the cassette, turned on the water pump and pressed the flush button and it worked! A lot of gunge came out but then the water would not stop flushing until I switched the pump off. Repeated this several times and same result.
Just wondering if it could be the Electric Valve Assey is faulty? It could be the original from 1994.
Any thoughts?
Thanks