My right rear caliper ebrake seal has failed on twice on my '83. The master seems fine, could this just be a crappy rebuild or should I look deeper?

 

Failed as in leaking or seized up? And the master what?

 

Failed as in brake fluid spraying from the seal. It was suggested that the master cylinder could be at fault but I don't see that being the case. Is there a proportioning valve in the master or somewhere between front and rear?