Herb, I have a piano here with exactly the same problem.  Small,
irregular lumps of a waxy green substance called verdigris have
appeared apparently out of nowhere between the pusher disc and inner
facing of each valve, warping and distorting the leather and causing
all the valves to leak a little.  The valve seats may be affected as
well, but usually to a lesser degree.

When you multiply this individual seepage times 88, it takes fast and
furious pumping to make much of anything happen.  The older secondary
valves (those with four screws instead of two) are notorious for having
these problems.

The solution is to disassemble each valve, clean thoroughly all the
metal parts, and replace each inner facing with new non-corrosive
leather.  By this time all the outer leather facings, if original, will
probably be turning to powder and need replacement as well, especially
those in the bottom row where there is greater circulation of air at
all times (including while the instrument is not in use).

When replacing the inner facings, omitting the felt backing and using
slightly thicker leather is recommended, just as Standard did on their
later production.  The advantage of extra thickness is less flexing as
the facing pulls away from its seat, allowing more precise regulation
of travel.  Don't forget to include just a tiny bit of wobble to
compensate for any lack of perpendicularity between the valve stem and
seat.

Jeffrey R. Wood