Check to see if the optional filter is stained
i Check to see if the red mark of the clogging checker is not raised
ii If the filter is clogged, replace the filter element with a new one

Four steps to replace of the optional filter element:
1, loosen and remove screws of cover
2, loose nand remove screws of up four screws
3, Replace the filter element, O-ring, and gasket with new ones.
4, Attach a new filter element with following the procedure above in reverse order.