I have used a large screw driver/pry bar the pry behind cap against the knuckle but that bent the cap on the back side. Now I just use a slim flat-head screw driver and hammer it under the rubber seal and slowly pry around the rim until it pops.
I did the same with a chipped up narrow chisle and hammer and still got it off pretty easily. Id put antiseize or some grease on it when replacing it so its not difficult to remove next time and dont forget the washer.I use a sharp wide chisel and a hammer.