Applying Clip Mask Brush Setting.

HI, I have a small problem when using the black or white brush to apply on clip masks.  If I choose, Show All on an image to apply a clip mask, then use the black brush to brush away the image,  the brush strokes don't appear to accumulate.  For eg, if I use a 50% black brush on a clip mask twice, you would expect the effect to black out the image in two brush strokes.  But this isn't happening.  Instead, it almost undoes the last brush stroke causing the image to reappear after the previous stroke.  How can i adjust the brush settings so each brush stroke accumulates the effect?

Thanks..