Making landscape alpha brushes

I’m trying to make landscape alpha brushes. But I’m having some trouble. The first issue is how do I import 16-bit textures for use with the brush? UE4 seems to convert imported textures to 8-bit which doesn’t work well as terrain brushes. The second issue is that the alpha brush seems to be extremely fussy about the brush texture itself. Sometimes it will let you use it. Other times it rejects it. So what is it looking for exactly?