I very much doubt the master cylinder has anything to do with the problem.
Rear calipers are a common issue. Just replacing the seals will not fix it and if paying for it to be done would work out more expensive than fitting new (without factoring doing the job twice)
New calipers are not expensive
https://www.classicswede.org/shop/re...lliper-girling
I would also take a close look at the flexi pipes and if any doubt replace them