OK, it's time to tell my YT18H story. You have to read it all to see how it's related to thom's problem
I purchased a mint YT18H with a 48" deck and two stage blower for really cheap because the previous owner said the transmission was shot. He said it ran great but wouldn't move at all. He had a reputable source look at it and had tried all kinds of things, including disconnecting the tow valve cable to get that out of the circuit. He was fed up with it and didn't want to put any more money in it.
I got it home, started right up and ran great, I put the pedal down and it sounds like the tranny is straining, I get a small forward jerk then nothing. Tranny sounds like it's loaded but no movement. I suspect the brakes. Spend lots of time looking into brakes to make sure they're not seized. Rule out brakes. Now I'm tinking it must be some internal seals leaking and he was probably right. I don't give up yet...
I noticed the fluid had been changed and there was ATF where the manual says 20wt engine oil. Thinking the viscosity of ATF is thinner than engine oil, I decide to flush and fill with 30wt (since I couldn't find 20wt) and maybe the thicker oil would work better....next day...after letting it drain overnight (not easy to fully drain one of these units) I filled it with 30wt and crossed my fingers...So it ran and I was able to pull it out of the shop. Got it down to the bottom of my driveway, turn... slowing... stopping... the thing wouldn't go back up the very small grade of the driveway. Argh. I thought I had made progress. That was all an illusion. Feeling very discouraged I parked it for a long time...and started long for a donor. Somewhere along the line I filled it with 80w90 gear oil with similar results.
Fastforward 2 years, the Ford is no longer a priority, sitting forlorn in the back of a shed, and a YT16h pops it's hood out in a picture of some tractors in a scrap yard on Kijiji. I call him up, go see it on New years eve day in the depths of Canadian winter and pay a bit too much to haul it home, with thoughts of returning to the scrap yard for more tractors in the spring. I get it home and next to the YT18 and notice it's not exactly the same. I knew there would be some differences since the 16 had manual lift, and the 18 had hydraulic, but I still figured I could work around. I pulled the tranny out of the 16 first, wasn't exactly easy. Got the 18 up on stands, started taking the wheels off, and, here it comes......one wheel was missing the square key that makes the connection to the hub!!!!! Duhhhhhhh. ABORT mission!
Took one of the keys from the 16, put the wheels back on, filled the tranny and went for one hell of a spin!!
We used the 18 all year last year to mow 2 acres at the cottage and then I took it home, put a cab on it and hooked up the blower. I used it all winter to blow my large driveway/parking lot almost daily this year. I now plan on putting the 16 back together this summer, since it actually cost me more, and getting some use out of it. Right now it was just an expensive key or a lesson learned.
Sorry about the long post, hope you enjoy the story.