Master Cylinder Keeps Blowing Rear Seal

[1974ChevyC30]

Alright I have a reoccurring mystery that I can't seem to solve. I have replaced my master cylinder twice once was a remanufactured napa unit and the other was a brand new napa unit. The remanufactured one replaced the "original" unit that also blew a rear seal but lasted somewhat longer. Before I get into detail here is how long each lasted:
"Original": Unknown number of miles
Reman: 1 Night
New Napa Unit: 400 Miles

I have replaced everything in the brake system including: wheel cylinders, calipers, most of the lines, pads, shoes, etc... everything except the vacuum booster. Keep in mind this truck originally had a hydro-boost system from the factory(in 1974). The new unit blew right after I got my 454's compression back up with a intake/exhaust valve tweak and carb float adjustment(she runs real good now!). I'm at a complete loss why this truck hates the vacuum boost system so much that it keeps blowing master cylinders. Any advice or ideas?