Clark C70D serial P680D-0749-9852 KF with ZF 3 speed transmission.

Long story short, customer bought truck brand new and transmission failed within 2 years, damage to seal rings and housing. No warranty was honoured.
So customer bought a new replacement transmission and its starting to deteriorate, has lost 3rd gear and then 2nd gear. Oil and filter changes done regularly and sent out fluid samples throughout, 1st year was good. Now into the second year and they were coming back with water in them, and the most recent oil and filter change the filter had plugged and collapsed. We're stumped on how water is getting into the transmission, all sealed and no coolant loss, trans. fluid not above normal levels. Most recent fluid sample came back with severe amounts of various metals.
Unit always starts off in 1st gear.
Has anyone had issues with these ZF transmissions?