I just checked the Grunge Brush Vector Pack 01 and the Charcoal - Feather and the option for pressure was greyed out. Is this because I do not have a Wacom connected OR do only certain brushes have the pressure abilities?
What Monika said.
"Pressure" option will always be grey out if the Wacom driver is not properly installed, even if a Wacom tablet IS attached.
I have had times I was forced to reinstall the Wacom driver to get the pressure option to become active in Illustrator.
Even with the tablet and driver installed, some brushes do not have a direct "pressure" option. I find it curious the "Bristle Brushes" have no "Pressure" option even though they DO appear to respond to pressure. Maybe someone can explain this to me.
The bristle brushes are different than any other brush. They work with a physics engine, analyzing all that input stuff by themselves. And just like your virtual car in Grand theft Auto will burst into pieces when hitting the concrete wall, the bristle brush will draw somewhat like natural brushes do by just analyzing all the parameters the Wacom driver sends to it.
I do have the wacom connected and I enabled pressure in the brush options. However, when I apply a pressure enabled brush to my artboard, I am not really seeing any difference regardles if I touch the wacom lightly or firmly and regardless of how much pressure I apply when painting a stroke.
Can you share a screen capture of your brush settings? Make sure you set a different "minimum" and "maximum" width percentage. The default is 100% for both, which would show no difference. Calligraphic brushes are a little different; for them you would set the "Variation" to something other than "0".
To make it obvious set it from 0% to 1000%
To see the difference use more appropriate brushes for pressure sensitive input like the calligraphic brushes and set the Diameter and Variation to the same value