Hi Jim,  Yes, your slider valves need to be resurfaced smooth using
very fine sandpaper on a sheet of glass, then wet the surfaces with
denatured alcohol and immediately sprinkle on dry graphite and buff
to a shine with felt.

This, however, is likely not the full reason for the poor performance
of the player.  My guess is that it was never fully rebuilt so you've
probably got leaky bellows cloth and flap valve problems, not to
mention leaks in the stack if that was never fully rebuilt.

If you disconnect the bellows unit from the stack and air motor and
seal off all the openings, it should hold vacuum over a minute if well
restored.  It's often difficult to find the source of air leakage
because it's usually an accumulation of lots and lots of small leaks
throughout the system.

Take care,
Bruce Newman