Peeero, el plástico también tiene mantenimiento, más concretamente, engrase, tanto o más que el bronce:
"If the valve does not have a specific lube port (zirc fitting or other) then close the valve, remove the hose, sponge out as much water as possible and use a soda straw to swab the ball inside with appropriate lube. You would have to check the manufacturer's recommended lube on the bronze valves but on Marelon, use any synthetic lube like McLube, Silicones and Teflons. Do not use WD-40 or any spray lubes and avoid petroleum based lubes. If the boat is out of the water then you can do the same up through the thru-hull fitting.
Reattach the hose and clamps, checking for any signs of fatigue on the hose and rust on the clamps, and then activate the valve to draw the lube around the seals.
Do this twice a year and operate the valves regulalry and they should last the life of your boat"
Y ahora la "google translation":
"Haga esto dos veces al año y opere las válvulas de manera regular,
y deben durar la vida de su barco"
Todas opiniones de foros de los "Estardosunidos", un lugarejo con experiencia
en el consumo de estos "plásticos", tipo marelon y otros propilenos reforzados.
Y yo sin decidirme! esto es un sinvivir... 
Me voy a embriaguar! 