Clutch adjustment – When is it required

I would say if your clutch works fine just make sure that you have no tension on the clutch when it is released mine has a return spring that holds the pedal up to the floor board so push on it by hand while looking at the pedal rods there should be about 1/2 inch min.of travel before the clutch rods start to engage the clutch.that is how my 2005  foton 504a  is anyway.and when running the pto see if it can run with the clutch half way released but the tractor drive line still clutched.

280 hours and you've only changed the oil once?  You're asking for problems.  The oils (all of them) should have been changed when the tractor was first put into service as the Chinese factories ship them over with crud oil in them.  You drain, flush and refill with new US-made oil before you run it.  Then the engine oil should be changed after about fifteen hours – that gets rid of manufacturing dross like sand, metal shavings, etc.  At 50 hours you change the motor oil again, also the filter.  After 100 hours you should probably change all the oils one more time.  After that, every 50 hours you change the motor oil and every 200 hours change the others.

Oil is so much cheaper than a rebuild.

The clutch should have been checked/adjusted when new, and again at 50 hours.  Do it now.  It only takes an hour or so to block up the side of the loader and remove the one tower to get to the inspection cover.

Did you adjust the valve lash and re-torque the head gasket at 50 hours?  If not, do it now.

Yep, it's long over due for a complete service job. However, as cheap as these tractors are just keep using it and buy another when it fails.