Mask Brush Not Erasing Properly

I usually do what Dave suggests, as well as turn off all settings in Brus Settings, and use a fully hard brush.  Even then, I am more inclined to use the Lasso tool with the Opt key to link points, and use Opt backspace (is that a Mac thing?) to fill with black.  Especially in tight corners.

If this looks too stark/abrupt (not sure how best to describe it), then set a tiny feather in the options bar — but don't forget to turn it off when you are done.  We have all been bitten on the bum getting caught out with a forgotten feather.  Goodness, that sounds bad!

Additionally could you please post screenshots with the pertinent Panels (Toolbar, Layers, Options Bar, Channels, …) visible? 

Make sure your brush is set to 100% opacity and 100% flow. Also make sure the colour you are using is full black i.e. RGB 0,0,0. Sometimes it may look black but is actually just a very dark grey which on a mask will allow some of the image to show.

Dave