Here's a quote from my owner's manual.
Diesel fuel often contains small quantities of
water which can damage the engine if not filtered
out. The fuel/water separator traps this water and
prevents it from reaching the engine. The harmful
water deposits must be drained from the
separator canister during normal periodic service
and maintenance to keep the fuel filtration
system working effectively.
The fuel/water separator is located in the rear
Place an appropriate container beneath the
outlet and open the water release valve several
turns. Drain any water deposits from the canister
until clean diesel fuel flows from the valve. Close
valve by hand. Do not over tighten
For Cummins ISB, it is on your daily pre-trip checklist. I drain mine every two or three travel days. I drain it when engine is not running and coolant temp is <120F. Why? That's the way we did it on the SSDG's in the Navy.
And, despite reading the Freightliner maintenance manual, and the FL operator's manual, can't find that detail referenced.
(Ship's Service Diesel Generator. SSDG. Usually Morse-Fairbanks OP's)
Fair Winds and Following Seas