My pre 2015 Mountain Buggy brake feels a little difficult to release.

As a moving part component , the brake should be lubricated regularly to maintain optimum operation. Your brake may require some extra lubrication if it is difficult to operate.

Vaseline or a similar silicon based lubricant is ideal and works well. The below link will take you through to the buggy instructions, direction of where this lubricant should be applied are shown on page 9. I
Buggy Instructions – brake lubrication on page 9
Lubrication is available as a spare part.