The Mountain Buggy duet, terrain and MB mini have a foot operated brake which holds the buggy in place.
These are indicated by a red foot operated lever on either side of the buggy, which are joined by a wire frame.
If your foot brake does not engage (or disengage) on either side as expected remove your rear wheels and check if the metal pin (foot brake wire end) is in place.
We have made this brake easy to repair or replace, to refit the complete brake wire, please follow the video below