On my 4150 it was smoked when I bought it it will spin without engaging pto at idle been like this for years doesn't seem to effect much besides someone's ocd. Can stop the spinning by applying light pressure to shaft when it's not engaged However if you have ocd shouldn't be buying used up tractors,I'm gonna post this up for anyone, anywhere that ever considers buying a used Kubota tractor, far away or even somewhat local...
Kubota's, especially earlier models with hydraulically actuated PTO's have an inherent issue and that is...
The PTO brakes on Kubota's consist of one or possibly 2 wet brake discs and they are prone to failure if the owner had used the tractor to run an implement like a brush hog or any implement where the implement kept driving the PTO when the PTO was disengaged.
Why I always use an over running coupler on my brush hogs (the pull type or the bat wing) because neither have a built in over running clutch and will drive the PTO until they wind down and that destroys the PTO brake that is internal in the tractor.
Easy to tell if the internal PTO brake is shot, just grab the output shaft (wear a glove so you don't get greasy) and turn it (tractor engine off, PTO not engaged). If it turns more than 45 degrees in either direction or just rotates freely, the internal PTO brake is shot and that requires some serious shop time to repair because it requires a split to access the hydraulically actuated PTO clutch pack.
It only takes a few times with an implement free wheeling and driving the PTO stub to destroy the PTO brake discs.
not a good design far as I'm concerned but it is what it is at least on older Kubota's. Don't know about the newer ones, just the older pre 4 units.

I did buy an overrun coupler for my 4150 cause it's like a 6-8" shaft extension if your buying used implements with shorter shafts or installed a pats or quick hitch and need a few inches.