It would probably help if you said what make/model head you had. Some are notorious for having pump issues, like the Jabsco units.
Some heads, like the inexpensive Jabsco units, are generally not worth re-building. Others, like the Raritan PHC or PHII units are very reliable and quite worthy of being re-built. If you have a unit like the Jabsco or even a slightly more reliable unit, like the WC Headmate 90 which has been discontinued, it might be worthwhile to upgrade your head instead of rebuilding it.
Raritan offers a PHC LBA or PHII LBA unit, which is basically the entire head minus the bowl and seat. You basically take one of these and swap the bowl and seat onto it, and voila, you have effectively a brand new Raritan PHC/PHII head for a lot less money, since the bowl is one of the heavier and more expensive elements in a new head. Think of it as recycling. SBO has the LBA units for sale.
BTW, I'd point out that Peggie Hall considers the Raritan PHC/PHII to be one of the best low-priced manual heads for boats. She's basically the doyenne of the marine sanitation industry...