Thanks ALL for replies !

Lovering valves as per hydraulics diagram are SPL ( proportional valve ) , SVL ( normal electrical valve ) and PCL ( pressure compensated valve or hydraulic latch as I correct thinking ).
Both of them I removed from manifold and checked physically and also by 24 and 36 VDC correspondently on the lab bench - they are OK.
Using Analizer I checked lift / lower POT4 : 0.6V - 4.1V - 7.7V , it also good.
I did a C4 calibration for lifting / lower - the system accepted that calibration.
Today I'm going to remove the pressure control valve from main hydraulic cylinder to see , if it contaminanted or not.
The truck was fully operational after I did repair but after a few days this issue with so slow lowering happened.

Thank you once again.