Dynamo is the only one who even came close to the right diagnosis. The air valve assembly (part #1203W on the attached drawing Raritan PH II parts and drawing
has finally worn out...you need a new one. It's not in the rebuild kit. You can try removing it and cleaning
it, but as old as it is, I'm not optimistic that'll work.
You'll find it on the front of the pump housing...it's a little square flat nut with a hole in it that looks like it's molded into the cylinder, but is actually threaded in. When you replace it, wrap the threads in teflon tape to prevent it from leaking.
Btw...There are two versions of the PHII marine
toilet--those manufactured before June of 1992 and those manufactured after June, 1992. Each has its own unique repair kit...only about half the parts
in each one are interchangeable, so it's very important that you get the right one for your toilet.
The earlier version, manufactured between August, 1981 and June, 1992 uses repair kit # PHRKII. The PHII toilets manufactured after June, 1992 use repair kit PHRKIIC.
Any mfr'd before August '81 are the original PH, not PH II. Any toilet old enough to be a PH has to be pretty much worn out by now, but if you insist on grooming a dead horse, the correct kit for it is PHRK.