Tumbler, I'll try and help but my Futaba set is old school so some of the names of functions might have changed.
I'm not sure if you have installed the servo yourself but the travel of the spool on the valve is quite small (the version for spring air gear is even worse!) so you might need to use one of the inner servo arm holes to allow you to use sensible ATV values. Set up the servo in the centre position with the spool on the valve somewhere near the middle.
Go to your gear channel and set the ATV so that you get full movement of the spool in both directions. Once youve done that, you can go to AFR and set the up and down positions (down will have the spool of the valve somewhere near the middle).
Set up a programmable mix. The master channel is whatever switch/slider/stick that you want to use to activate your brakes. The slave channel is the gear channel. Set up the mix so that it is turned on with the gear switch in the down position (this means that with the gear up, the mix is off and the spool doesn't move).
Set your mix values so that when you activate your brake slider (or whatever you use) the brakes come on. Depending on the setup, you may have to set one direction of the slider to 0%, otherwise you can move the brake valve spool towards the 'gear up' position which you don't want.
I'm sure there are other (better) ways of doing it but this has worked for me. This has been written from memory so I apologise if I have got some of the steps wrong but hopefully you get the idea.