It's many decades since I flew a glider, evidenced by the fact that my ab initio instructor at Lasham was Derek Piggott, but I do recall "brakes in and locked" as an essentiial check.
I've come in though, only to go back a little way in the thread to a post which said:
I always started aerotow takeoff roll with airbrakes open and closed them when I have good roll control.
and ask; "Why?", out of sheer curiosity.