Hello 
This seems like a dumb question, but i just cant find a solution to this problem…
When i change the a Brush in one of these slots to “MaskRect” (or MaskLasso, or Smooth, just any brush who is giving an hint-layer which says that now i have to use the shift or ctrl button), i cant choose another brush for this slot!
This is somehow annoying, is this a bug?
Thank you in advance!
If you hold Ctrl, Shift, or Ctrl+Shift together, different actions can be performed from ordinary sculpting:
Ctrl = Masking brushes
Shift = Smoothing brushes
Ctrl+Shift = Selection, Slice, Clip and Trim brushes
Because there are several different brushes for each of these types, selecting one of them in the Brush palette will make it the default when you hold the relevant key combination. So, for example, if you select the Mask Lasso brush, when you next hold down Ctrl the Mask Lasso will be selected. To change it, all you need to do is select a different masking brush and that will then become the new default when holding Ctrl.
HTH,