Our 1983 Scamp
16 has the Sealand 911 also. I was able to get parts at my local Camper's World. I think Sealand and Thetford are the same company now. I'm pretty sure that they still sell the 711/911 toilets.
The manual I've attached is for the Sealand 711 but it's the same as the 911. The '800' service numbers may still work.
I would just take it apart and clean everything and then reassemble and see if it still leaks
down. Mine had stuff that was preventing the ball from rotating completely back into position.
IMHO, these are a top of the line RV/Marine toilet, fairly simple, pretty reliable and easy to repair; unlike the cheap marine toilets (my other camper's a boat) that you're better off throwing away and replacing.
Note to Moderator: This manual should be added to the document center .