I would image, given your problem that the caliper bores are worn-out beyond
tolerance. 1970?, they're nearly 40 years old! Rebuilding brake components
is false economy. Someone further into this thread referenced the U.S Army
(Silicone Brake Fluid - BFS), The Army banned rebuilding of hydraulic brake
components in 1979. Back in the day, with lower speeds, less autos on the
highway, and not nearly the amount of aggressive driving as seen today, it
was a common practice to overhaul brake cylinders and the like. Not worth it
anymore. Then there's those that say "I want keep my car original and
authentic". Well, if you put more emphasis on aesthetics than you do
safety,.... please get off the highway, as I have several grand-children
that have enough dangers to worry about in this crazy world without having
to worry about some nut with shade-tree brakes on their car.
Think about it. If the seals did stay in the bore, and you got out on the
road, had to apply the brakes in an emergency, and THEN your seals
popped-out, what would you do then?
You wouldn't put rebuilt tires on your 'vette, so why the brakes?