Hello,
I want to make just a circle. I forgot which option gives this possibility. I know that it’s something about to set the number of corners. So by that the shape of brush get changed. A big number would affect in circle then AFAIR.
Tried to search, but no effects 
Greetings,
Igloo
create a brush the size you need, then select CURVE then CYLINDER to
create it, if all you need is the flat circle, cap the cylinder then delete it (cylinder ) leaving the caps ( 2 of them ) then delete the cap you dont need.
Or move the second cap to the same place so its double sided.
Patches aren’t perfectly round though.
If you want a flat cylinder (like a coin) then keep the other patch and stretch it.
If you want a flat cylinder as a brush, theres a create brush with a specified number of faces.