lost the hydraulics on my D 15

        Last fall when I went to unhook the hay rake from my D 15, the hydraulics  wouldn't work to raise the lift arms and run the single acting cylinder on the rake. I was in a hurry, and just assumed that it was low on oil, so I left it go for what was supposed to be for a couple days, but didn't get back to it until today. Today I went back to it with some hydraulic oil, thinking that I'd just add some oil and it would work fine. Not so. When I pulled the dip stick, it was not low on oil, and still no hydraulics. The lift arms raised very slowly, then the rake started to come up, but that was it. The best it'll do now is just barely raise the lift arms over about a 5 minute time period. I added some marvel mystery oil to it and drove it for about a half hour thinking maybe there was a valve stuck that would free up, but that didn't help. I know this question has probably been asked on here a hundred times, but I'll ask it again, since I'm not too computer sharp at looking for posts from the past. What could the problem be? Any help would be greatly appreciated.   Thanks, Darrel

Do the arms go down using the lift/lower lever and how does the sound of the pump change with the clutch in or out?  Also look and see if the block that adjusts the traction booster is loose at all, I thought my pump was shot until some guys on the forum told me to adjust the traction booster, that block is real sensitive to adjustment.