I would like to offer this for John McClelland's consideration.
On Ampico grands, the location of the equalizer and space available
determines the length of the spring.  Often the treble side is much
shorter.

The purpose of the equalizer is to smooth out surges at minimum
intensity playing.  The equalizer is a small reservoir which keeps
the vacuum level from dropping below minimum operating levels.

For the equalizer to function properly, it should be almost fully
collapsed when the piano is at its softest playing vacuum level, with
no notes playing.  When system demands cause a drop in vacuum, such
as several notes being struck at minimum intensity, the equalizer(s)
quickly open, and dampen the surge.  Obviously, if the equalizers are
set too far open at minimum levels of play, they will also dampen out
all quick, subtle accents.

Any spring will work if the equalizer is kept in the active range
explained above.  Differences in springs between the bass and treble
will have no effect on performance just as long as each equalizer is
nearly fully closed at the softest level of play.

Bob Taylor