Read the following maintenance procedure: 'Procedure 4.1: Filter Replacement.
CAUTION: Ensure system is depressurized before proceeding.
Step 1: Place a drain pan beneath the filter housing.
Step 2: Using a 24mm strap wrench, turn the housing counter-clockwise to loosen.
Step 3: Remove the old filter element and O-ring.
Step 4: Lubricate the new O-ring with clean system fluid before installation.' According to this procedure, what must be done immediately before installing the new O-ring?
