The SeaLand 752 was discontinued in the late '80s...no parts
or any other support have been available for it for at least 20 years, so I'm guessing that all the rubber parts in it are completely worn out by now. I'm amazed that it's still working at all...It's definitely passed time for a new toilet!
This is the only manual toilet made today--the only one still made in the US--that's anywhere near as durable: Raritan PH SuperFlush
It's a available as both a complete toilet or, if his existing bowl, seat and lid are still in good shape, a "conversion" (everything BUT the bowl seat and lid...and yes, his bowl WILL fit).