It's an open system, so fluid flows all the time, only a small portion of the fluid goes through the cooler.
There are 2 loops to that system one is the HST loop and the other is the Hydraulic loop they are shared.
How does the three point lift do?
How well does it move?
Suction is on two lines one going to the main hydraulic pump and the other is from the case to the bottom of the HST unit.
Follow all the line and look for seeping leaks, wet spots.
The hydraulic fluid running hot is not that unusual, they do run hot.
Get a temp gun and get some actual temperatures to help with determining if you have a real issue or if it is within normal running limits.
Also because of the alterations to the engine, I'm curious if they have altered the hydraulic system or cooler too?
Only alteration to the engine is the alternator addition.
As for how it moves, some days there is an issue right from startup where it just refuses to move, makes absolutely no sound and behaves as though you are not even touching the pedal.
Some days it'll run for a reasonable amount of time then randomly stop, lowering the revs right down will often bring drive back.
Other days it'll run fine right from the off, but after about an hour (or less if pushing through brush etc) the HST unit and lines will get hot and it'll vibrate like mad through the pedal (almost unbearable to hold your foot on) and drive line until it stops and then behaves like as if you're not touching the pedal, the only way to bring it back from this is to wait until it wants to try and creep again, then knock it out of gear, push the HST pedal either way and leave it that way with the revs up until you can feel the lines getting cooler, often this can be 10minutes.
as a bit of background info, I bought it knowing it had a fault, and knowing I could get my money back.
When bought it would only drive a few feet for a few minutes before stopping dead.
Investigation showed that at some point it has had the wrong fluid put in, it had ATF fluid in (which was clean) filters didn't appear to have been changed for a while and hydraulic mesh screen had a 'paper mache' type substance on it, HST screen only had a small amount on. draining the fluid that was in it revealed what I would deem a reasonable amount of it in there.
No metal filings at all in the oil and opening the filter also revealed no metal filings.
New genuine HST filter and Kubota UDT fluid and it is much, MUCH better, but still incredibly annoying when it stops because once it's stopped once in the day, it'll most likely keep doing it.
3pt links work really well, but will drop over time (quicker than I'd like)
I asked about the 'plumbing' of the machine because if you follow the line that comes out of the top of the HST into the oil cooler and then out, it goes into the same hydraulic block that goes to the 3pt link, there doesn't seem to be any obvious cooling circuit from the HST unit back to the reservoir.
As for leaks, there is nothing, no weeping and no wet spots.
If you do a search of my username on the forum I have asked about this HST fault last year and there should be a video too.
Thanks.