Always use DX for battle important operations. Pickle, trigger, brakes etc. for T/O, landing, cruise stuff, you can use keys.
The problem in this case is that you are using a modifier to operate the brakes (shift, ctrl), so when extending the brakes you are pressing shift, which is also added to SPACE, making the game see L_shift + SPACE, which in this case doesn't do anything.
Modifiers are global and are added to any keypress or DX or whatever across all controllers. So pressing shift, ctrl or alt, will change all controllers to a different layer.
Hope that was somewhat understandable.
There is a 120DX t.a.r.g.e.t file in the Thrustmaster section, that will give you more than enough DX buttons. I suggest you download it.