There is a black box (brake activation switch) on the frame near the coupler. The black pin goes into that box. Once you reinstall the pin the brakes should release.
Check out the beginning of this video:
Lee and Anne
2016 F250 2WD CC SB XLT 6.2 3.73 3,295 lb payload cap.
2014 Salem Hemisphere 282RK 8k lbs loaded, Equal-i-zer WDH, TST 507 TPMS
2016: 33 nights camped; 5 nights booked