This gasket fits into the underside lip of the resevoir cap.  The last thing you want is brake fluid dribbling down and getting onto that hot manifold.  Not pretty and not safe.  Make sure that your caps are tight! Please!  The filter portion snaps off the lid and the portion that the filter snaps on to also pops off with a slight prying of a slotted screw driver.  Once both pieces are off, you can change out or add a gasket.