There is only 2 things that allow a seal to leak, the seal its self
and the surface it is sealing on. If you have been around the Corvette
groups very long you have seen the problems with brake seals, same
problem. Rust. The yoke is cast iron and is polished by the seal when
driven. Since it works in and out as well as around it can be left set
with bright polished cast iron exposed to allow rust to form. Next
time you drive it the rust becomes a rasp and eats your seal leaving
some rust in the seal to eat the yoke. At some point even a new seal
will not seal a yoke with a ring cut in it from the previous seals.
Some repair shops have the ability to sleeve the yoke, most don't
and/or choose to replace it; but either way it needs to have a good
sealing surface. Is it common? Yes, for cars that sit for long periods
of time regardless of the transmission type. As for the brake seals
they just trap the moisture under the seal a eat the sealing surface.
Very good reason to flush the brake system as required for its type of
use.

As far as you injector problems, when were the cleaned last?