Could be piston, valve body, hose, or cable adjustment(way off). The pump is most likely OK, if it can lift the blade. Once it is lifted, it is the valves job to hold the pressure in the line, to keep the blade up. Any leak will cause it to drop. Is it a electric/hydro setup or the belt driven pump style. The more info the better...
The website for Fisher is http://www.fisherplows.com/ They don't really have a section at their site to pose technical questions. The site tells you to talk to your local dealer about service questions. You can find your local dealer through the site. I contacted the Fisher "experts" and they weren't much help. They don't even offer a service manual like I've seen for other plows. Stick to hitting your local dealer with the tech questions.
Probably not. That's a single acting cylinder, if the leak was internal to the cylinder there would be oil coming out around the pushrod seal when the plow leaked down. If ti was double acting you could have leakage around the piston itself and never see it on the outside, but not on a SA cylinder. Check the cables first, make sure they are adjusted to allow the valve to close completely. If nothing else, lift the plow and unhook the control cable at the valve end. If it keeps dropping, problem is in the valve, if the settling stops then the problem is in the cable, which is holding the valve open slightly.
Being 12 years old, my guess still is the piston. All the ones that I have owned have creeped, a quick rebuild and they no longer creeped down. You can try adjusting the cables at the valve body, but that is very tedious, in frankly unless the plow is dropping to the ground every minute, not something I would worry about to much.
Dino,, no need to get huffy when someone disagrees with you, but you DO seem awfully mature for being only 12.
Anyhow,, single acting cylinder, one hose in and out. Only other place oil can get out is around the pushrod. If the rod is extended and it leaks down the oil has to go either back out the hose (bad valve or leaky hose) or weep past the internal seals and show as a leak around the pushrod seal. Most likely the pushrod seal is not intended to hold pressure, merely acts as a wiper to keep crud from getting in the cylinder.
When you had cylinders rebuilt, were they leaking around the pushrod? If not, where was the oil going? I know that theory says it either has to go back out the hose or daylight at the pushrod, so I'd like to know what I'm missing here.
Mine always showed some leakage around the the rod. He didnt say if that was the case or not. I very well may be wrong, however I will repeat, that unless the piston is dropping at a fast rate, I wouldnt worry about it.