My brother's got an old sand filter with the push-pull type backwash valve off to the side (Pentair?). The valve is leaking and won't go all the way up to turn it into the 'locked' position. I'm guessing the tank is probably 20+ years old. Its is made of fiberglass and the surface layer of the resin has worn away over time and the fibers are exposed. I don't think it affects the integrity of the tank, but it looks bad and if you happen to rub against it, you get stuck by the fibers.
The filter seems to still work and I can replace the valve for ~$70. For the cost of just the valve, I'm almost 1/3 into the cost of a new filter. I'm debating whether or not it's time to change the tank out or if I should replace the valve and sand and call it a day.
I know with pumps, bigger is not necessarily better, but it seems that this would not be the case with filters. If I replace the tank, is there a formula to calculate filter size? I don't know exact capacity of the pool but I would guess it's 18k-20k gallons. Is Pentair a good brand? I've got a local dealer that sells them and stocks replacement parts.