I thought it would be a good idea to post a refresher course on changing the oil in the yt's hydro. Although I have the 6700 some might benefit from the info along with the pictures.
1. Jack up the back of the tractor so both wheels are off the ground, brace so it will not roll.
2. Remove the left rear wheel. 19mm socket
3. Remove the clutch connection pin and lower the assembly.
4. Remove the bracket that holds the pressure relief rod and lever, two 12mm bolts.
5. Remove the center nut, 14mm with the push pin on it.
6. The oil will start draining, forgot - good idea to have a drain pan under the unit!
7. While the oil is draining it's a good time to check the level of the gearcase oil on the differential. 13mm.
8. Reverse the proceedure and refill the oil with 10w30 (recommended) I use 40 weight.
9. It will take some effort to get the oil passages to bleed the air out, do this by manually pushing the tractor back and forth about 8 - 10 feet with the engine off, do this until you feel the wheels lock up. After starting the engine gradually coax it back into movement until you are confident that all is well.
I put a generous amount of anti seize on the wheel shaft for easy removal next year.
I also changed the hydro oil in my YT3600 and will add a little more info that may be helpful. In case you're a newbie like me that doesn't have an owner's\service manual.
On the YT3600 the relief valve is on the right hand side. The lever you pull up under the seat to push or pull the tractor when the engine is not running is what engages the relief valve that you will be taking out to change the oil.
I removed the seat and seat pan(as I was doing engine work and had to remove the shaft to the hydro) and found the access was very easy for changing the hydro oil. But you can probably also remove the right rear tire and access from underneath...
-remove two 12mm bolts that hold the relief lever bracket
-remove the relief valve with 9/16 wrench (everything else is metric but this is 9/16...go figure)
-pull out the relief valve and if you see it the little spring. (spring may not come out but don't lose it in the oil)
-I rocked the tractor back and forth as it seemed to push more of the oil out of the unit.
-replace everything, fill with oil and rock tractor until you feel resistance then should be good to go.
Also...I pulled the engine out of my tractor to replace the front gasket and front main seal so if anyone has any questions regarding that I may be able to help some.
First I am not trying to step on any ones toes,I am just a backyard mechanic but one that loves it.
I just want to mention, (and I have before two or three seasons ago),that I did change the hydro drive oil in my 3600 and its nothing close to what I am reading in here.
I am not and cant speak for any other model.
All I remember having to do was, (1)( take off the seat and remove the cross bracket(2) I had to look very closely but finally spotted a flush mounted set screw that needs an Allen key to unscrew.
(3) With the fill cap already removed to allow proper drainage I removed the screw and all the oil drained out all over everything (forgot the drain pan but it would have not caught it all). I didn't bother to flush some new oil through but may have been a good idea since this was the first oil change in 14 years. (Previous owner told me he only changed the engine oil.) I have read more than once how you have to remove the auto stop safety rod etc. I have no idea why its necessary. (4) I replaced the drain screw and filled it up with new oil, (I did rock the machine slightly to make sure it was full). (5) I replaced the cap started the machine ran it gently in forward and reverse checked the oil level again and it was still full.6) replace bracket and seat.
I have used it for three summers since and its just as good as new.
In a nut shell it was remove seat and bracket, take the fill cap off,remove drain plug and drain oil, replace plug add oil and replace the fill cap, replace the seat and bracket, and your done.
Now I regret not posting pictures but I didn't take any, that being said I would be more than happy to do it again and take pictures as its been three years so it wont hurt plus I need to change the differential any ways.
There is a lot written about air traps etc but I have never had to add one drop since I filled it, I cant swear this was the standard or right way but it was fast and it worked.
I have the YT6800 (think that is right #) Unfortunately it ran out of fluid in hydro static trans. I replaced the oil with 10- w- 30. It will now only move a few ft in forward and reverse. Can you help...thanks ian