Quick correction to Bill's post: an Adventure Pass is not required to park at the Manker Flats trailhead. It's not listed as one of the "Standard Amenity Recreation Fee" sites in the settlement agreement in the Fragosa case where an Adventure Pass would be required. Also, it's specifically identified as a "non-fee" site on the Angeles Forest Recreation Fee map (where it's called the "Baldy Bowl Trailhead"):

http://www.fs.fed.us/portaldata/r5/ap/map/anf.html

Hope this helps!