I did a search and found the site below. It seems they offer a rebuild service. One of the posters at FEP just tried to refurbish his and had a really hard time. After seeing the heartburn that fella had I'd give them a call. He also bought all his "parts" from MCR but it turned out there were pieces needed that weren't in the kit/not available, etc.
Brakes! Old Auto LLc -
Old Auto LLc - Caliper and proportioning valve rebuilding service for antique classic cars and trucks. GM, Ford and Mopar brakes.
oldautorestorations.com