On my 855 I'm thinking it's under the seat on the right side looking from the front of the tractor..... been 19 years since I've had to adjust it. Others here that have your machine they might be able to help you.
There are two set-ups you can have to control the deck lift of a MMM on a 2305. The standard lift is done by what Deere calls the mechnical lift. It uses your three point hitch. The second is done by an option known as the indepenant lift, which is a seperate hydraulic cylinder controlled by the joystick. Both of these will leak down over time. The mechanical lift will take the longest to "leak down" and can be locked up with the control knob directly under the seat.
The independent lift can leak down very quickly, less than five minutes. If you want to keep the deck in the up position you will have to turn the control knob to the lock position. The deck will still "sag" some but not to the ground or a cutting height.
Unfortunately, this is normal and they all work this way. Hope this helps.
A customer complained about the quick leak down of the 2305 mower deck so I verified his problem and submitted a DTAC case on the issue. The reply was that this was normal since the hydraulic cylinder was a single action type. This explanation was totally unacceptable to me. The cylinder should hold for more than 5 minutes whether it single or dual action. The customer returned the tractor with the JD 30 day warranty.
My SIL bought a 2305 with the rockshaft lift and there is no problem with his mower.
Like the others stated, this is a problem with the IL (independent lift) option. JD's position is that it is only for lifting the deck-you still use the knob to set the cutting height and to hold the deck up. I personally see no point in the IL kit since I never have or want anything on the 3PH while mowing, it cost's more, and it's more complicated than the regular mechanical lift setup.
I wonder how these new 1000 series machines are going to differ from this tendency, if at all. I sure hope JD put some thought into it. That was one of the things that drove me crazy with my 2305/54C with independent lift. They could have at least made the detent on the height adjuster that keeps the deck raised closer to the raised height of the deck. I think it used to drop a good inch or two or three, almost pointless. I always wanted to get under there and weld on a hex nut or something to the height adjuster to keep the dang thing where it was supposed to be.