Time for a hew toilet
The 59128 was the model # for the "Bryden Boy"--a Canadian version of the Jabsco manual made in Canada under a licensing agreement. Bryden went out of business in the mid-'80s...so yours has to be one from one of the last production runs, and original to your boat...making it at least 20 years old. Jabsco continued to offer a service kit for it, but discontinued it about 5 years ago...no other parts have been available for at least 10 years.In ALL toilets, the joker valve is located in the discharge fitting...it's a cup-shaped rubber one way valve...the flange serves as the gasket that seals the connection between the discharge fitting and the pump housing. The current Jabsco joker valve should fit it, but replacing it won't solve your problem...it has nothing to do with bringing flush water in, it's a backflow preventer in the discharge. My guess: the flapper valve at the top of the pump has failed. Every other seal, o-ring, valve and gasket in the pump has to be on its last legs too, if it hasn't also failed by now. So it's time for a new toilet. Andy's description of the current Jabsco manual toilet as "disposable" is right on the money. 15-20 years ago they made a decent product for the price...but nothing they've made in the last 7-8 years is likely to last more than a year or two, if that long. The Raritan PH II--or PHC, which is the same pump, but on a compact base to fit in a smaller space--is the best choice for a replacement. It costs more--about $275 if you know where to shop--but is the only toilet under $500 that'll give you more than a few years of trouble free service. Keep it lubricated, rebuild it every 5-6 years, and it'll last for at least 20 years. It also pumps twice as efficiently as any other toilet in its price range. You can check it out here: http://www.raritaneng.com/products/toilets/manual/ph_2.html