Genuine AP Original Mk3 Cooper S and 1275GT servo is now supplied as this universal kit for single line brake systems. This kit replaces the servo originally sold on its own at no greater expence, but the brackets are universal for fitment to any vehicle and not the originals for Cooper S and 1275GT. Known as No6 type. 
A servo on drum brakes does wonders for stopping and if a future disc brake conversion is planned, a servo is a good first step. Air valve should be at bottom or 30-45 degrees up from bottom.