Use keyboard for accessing Emacs Menu

I am using the

brew install emacs

version of gui-based emacs. After using the mac shortcut to access the Menu bar, I am able to use the mnemonic to access the individual top level menu items: e.g.

  ALT-F  for file

The problem is that then hitting down arrow key or return does not dropdown the menu items. Well .. with two exceptions: the

Alt-E (Emacs) and 
Alt-H (Help)

dropdowns actually work. The others? Nothing happens. Only by using the Mouse (which is unacceptable to me for common commands in an editor).

Any hints/ideas on this?