Chester,
That is a check valve and not a secondary fuel filter. The following is from my DD Series 60 manual:
"An optional PRO-CHEK check valve that removes air from the fuel supply line may be installed between the secondary filter and the cylinder head. The fuel flows to the injectors in the cylinder head through passages intregal with the head. Surplus fuel exits at the rear of the head just above the inlet, through a restrictive return fitting which maintains fuel pressure in the system and returns back to the fuel tank."
In further reading the PRO-CHEK is only replaced when it is leaking, cracked or other signs of damage.
__________________
Steve
Newell
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