Yes a valve is there, but it's either very touchy across a smaller range than I've been trying, or I have something else out of whack.
I've stopped dealing with the outfit from which I bought the tractor because they seem much more interested in sales than service: below the seat are two levers, one of which would not latch [seemed not to have a detent] into place (it admits hydraulic pressure to the rear hydraulics), but the dealership installed a modified turnbuckle (before initial delivery) as a fix. It seemed weird, at first I thought maybe it was something their experience suggested was helpful: I'll claim newby innocence, so don't jump my case.
I gave up complaining, and when I returned for an oil change (I know I shouldn't have), they acted horrified and fixed the situation. I stayed loyal too long, but I'm not going back.
And by the way, LS manuals stink.