I started Brakk farming yesterday at 10pm, and got the final part today at 7pm.
I went to a total of 50-55 invasions, and they spawned 10 times.
I key to make them spawn is to take it slow.
Every time i was with a group that sped through the map, they never show up,
but when we took it slow they showed up 1 out of every 5 invasions.
I calculated the best way to go about it, and if you wait in the start room for 1 minute,
then go through the level, they spawned 1 time every 3-4 missions.
Not a big spawn rate increase,
but I managed to get all the Brakk parts from start to finish in less then 24 hours.