Snacko, AFAIK there has been a recent change in input. Speed brakes on the F-86F are controlled by the three-position speed brake switch on the throttle. We now have three input commands for the individual positions (OUT - NEUTRAL/HOLD - IN) and two separate toggle commands for moving the switch up / down.
The cold start mission was the first training mission and has not been updated yet to reflect this change. Other missions (A2A Guns, for example - see screenshot) already incorporate the new behavior.
EDIT: Sniped - badly ;)