Naaa some have them some don't and those that don't never suck in their rubber. I've never been sure what it does - whether it stops the cylinder spitting back at you if you bleed the brakes with the cap off or some other dark Ju Ju. Needless to say if it has one I leave it on.........
Thanks pkay. @maelstrom might be on the right track as there is a tiny little hole under the "beaver tail" flap. The flap doesn't rest against the hole, so might be to stop blockage from something above it. I'm rebuilding a master cylinder for the first time, so keen to understand all the "bits".
