make sure selector is in flush position and not pump dry. in pump dry position the intake check valve is defeated by an arm that holds the check valve open - thus no suction.

arm my be seized on shaft. should move freely

pay attention when you take it apart, maybe take pics. as i remember it can be reassembled incorrectly.

rebuild kit currently available from Fisheries Supply in Seattle, search: wilcox-crittenden-seaclo-toilet-repair-kits
probably others. about $80. kit has all replaceable parts included. pic is bad, may look different that what you have, some updated parts.

mine works like a champ after rebuild. all fasteners [stainless] released easily and went back in with good holding power , stripping. straight forward. pump can be removed from head with out removing head from boat. have wet vac handy, bleach cleaner. there is a drain at the right end of the horizontal tube to remove almost all liquid.
good luck.