In regard to leaking Ampico unit valves, I assume you are using leather
valve facings, and the leak in which you refer, is the mating of the
leather valve top on the seat of the metal top?

First, one should not test valves at such a high vacuum level.  You get
a better idea of leakage at lower vacuum levels such as 10" or less of
water vacuum.

A *very small* amount of leakage (actually seepage) is to be expected.
Ampico knew this, and compensated for it.  However if the leakage is
large, you may have bent the top valve seats slightly when removing
them, or did not seal the edges properly (with shellac) after gapping
them.  I use thick shellac, which is regular shellac, allowed to
evaporate a bit, or burn off some of the alcohol.  Do not seal with any
type of glue, phenoseal, or you will never get the valve out again for
rebuilding or adjustment.

Bruce Clark