Interesting thread!
OP has not posted back.
I'm thinking he found coolant leaking into the heater core was the problem.
He seemed well versed in AC repair!
Perhaps the OP is finding some of the "fines" left over from Kubota's machining processes?
Many have noticed metallic residue when draining the front axle,as have I?
Are you sure all the glow plugs are good?
I remember those 2.5mm fabric covered hoses!
Used to keep a meter of it around.
Even used it to repair my 6.5 GMC diesel return lines.
Have yet to hear of this being an issue with Kubota engines.
Good Luck!