I am attempting to make a brush, and I would like some assistance in getting the effect I want to achieve.

I want a brush with large spacing and low opacity that every time I draw it back across itself on the same stroke, it will overlap/ get darker/the opacity will compound.

I can achieve a similar effect by making opacity dependent on pressure, but I would really like to be able to achieve the effect by compounding opacity in a single stroke.

Does anyone know how to do this?