You can't unless working with 100% opacity brushes.
This is how Photoshop works.
In case you're unfamiliar, to do this you need to change the brush settings (Window > Brush, then go into "Transfer" or I think it's called "Other Dynamics" in CS4 and earlier) and set the "Opacity" setting to "Off" (it can't be "Pen Pressure" or you'll get the overlay effect). Have the "Flow" setting to "Pen Pressure" instead.
Also, try turning off "Shape Dynamics" and setting the spacing to something like 9%.
Then when you're painting, leave the opacity at 100%. You can play with the flow setting, start it at 100%, some like to change it down to 30 - 70% as they paint depending on how soft or thick they want to lay down their colors/values, it's up to you, just play around with it and see what you like.
Apply these settings to a hard round brush to get the control I think you are looking for.
Last edited by landylachs; April 2nd, 2012 at 06:17 PM.
Reason: added some info