While we are on the subject of manual heads, a maintenance suggestion.
You have probably been told and read to flush various kinds of vegetable oil to "lubricate" the parts and to keep the rubber bits from "drying out." Do NOT do this. It has been repeated so often it is accepted as "true", but that doesn't make it so.
It has no beneficial effect on the rubber parts, and is a rather poor and short lasting lubricant. Oil can contribute to clogging up the holding tank as it saponifies into sticky gooey fatty acids. Leave the vegetable oil in the galley where it belongs.
Instead, remove the pump plunger. This is easily done on the toilets with the locking handle, use push down and unscrew the top of the pump. Lift out the piston assembly. Put a very small amount of silicon grease on the o-ring that sits on the rim of the piston, a tiny bit more on the stainless steel shaft, and one more spot on the o-ring on the fitting you unscrewed. Using more than a thin layer will work no better and last no longer. Put it all back together and be amazed at how smoothly it works!
Repeat anytime the pump starts to feel stiff. On our boat, in daily use, we find this is about a month or so. Your o-rings will last a lot longer when they are properly lubricated.
For silicone grease, I use Dow Corning Molykote 111
A 5 oz tube for less than $20 will last two forevers on a boat, it is good for every rubber o-ring and seal you have. It is the same stuff you can find at plumbing suppliers and dive shops repackaged into smaller containers.
Back Creek, Annapolis, MD