Two most likely causes
If you're not feeling any backpressure, the flapper valve at the top of the pump has prob'ly failed. If you're feeling backpressure--as if there could be a clog in the line--the dry/flush valve has failed. Either way, if the toilet is more than 5 years old, I wouldn't put ANY more money into it...I'd replace it. 5-7 years is about the max lifespan for a Par or Jabsco toilet. And as long as you're gonna replace the tank AND the toilet, I'd go ahead and replace all the hoses too. Unless the leak in your tank is at the top, I wouldn't use the head again until the new tank is in. With the weather warming up, the LAST thing you need in your boat is a sewage spill. For the best tank source IMO, read "What Size Holding Tank Do I Need" in the HM formum reference library. You might also want to read "installing and head and holding tank" and "replacing hoses" before you start this project.
