I tried to contact the Power Brake Booster Exchange and nobody answered the call. I also contacted a brake rebuilder in Minneapolis MN and they didn't have anything for a 1971 Demon. I did find some help at Master Power Brakes Inc. Their web address is info@mpbrakes.com and their number here in the US is 800 472-4181. Look at their website and view part number BM13203-1971-dodge-demon-bm. This is a booster/master cylinder that would probably work on your car. You could also go to a manual brake conversion (fairly easy to do) and get rid of the vacuum booster. I has a 1974 Duster with manual brakes and it stopped just fine. Hope this helps!