Pugxp6t

Hello folks!! I picked up a Pug XP6T for my boys 16th birthday and has had a problem with the rear brake not pulling any pressure to work. We've tried bleeding the system, back bleeding etc etc.... Being the model it is we are finding it very hard to find parts to replace, so am wondering if we can just put a new full rear brake system on it, or new aftermarket rear calliper it Any other suggestions much appreciated. Cheers Puglife