My bad. Now that I re-read your original post, it does sound like your "lowering control valve". I read it as your 3-point is going down on its own.
If it's going down slow it's most likely that adjustment. If there is no implement on the 3-point, even with that valve opened, it's common for them to go down slow due to weight. If you want them down so you can connect an implement, just push them down with your arm (so long as that valve is somewhat open).
That valve/knob we're talking about should be adjusted based on the implement you have connected. If you don't adjust, a heavy implement comes crashing down while a light implement takes forever to lower.
Once I connect the implement, I hop in the seat and lift the implement. I then reach for that knob and close the valve all the way. I lower the 3PH control arm (to the right of the seat) all the way. I slowly open the valve between my legs until I like the speed the implement is lowering then leave it there..
I just grabbed this pic off the internet. I see 2 knobs under the seat. The upper looks like it's for some sort of seat adjustment. If so, I'm guessing the lower is the knob/valve we're looking for. Hopefully once you get in close and wipe the dust off, you'll see something on the knob or a sticker nearby that says what it is.