Gettin' my head straight

My aft head, which is manually pumped, has had a problem for a while. It will flush away water/waste/etc but it won't refill itself with seawater. I did a little research, since I didn't know anything about marine toilets, and I found that it most likely is a bad flapper valve (or top valve gasket, as Jabsco calls it). I bought one for around ten bucks and pulled the toilet pump apart.

I also grabbed some of this stuff. I don't know if it will help, but it certainly can't hurt...

The flapper valve did look pretty bad, so I replaced it and reassembled the pump. The good news is, these toilets are incredibly simple. The bad news is I still wasn't getting any flushing water. I checked the inlet hoses and they're all clear, so it looks like this guy is the culprit. I'm not positive that the through-hull isn't clogged, but with a flashlight I can see that the valve isn't opening when I turn the handle, so apparently I've got a faulty seacock...

Everything else still works fine, ladies