Lorn, they all suck harder than required (when new). You need a regulator
between your pump and gyros to reduce the pump output to the gauge
green-lines. It's standard equipment on every airplane, using
vacuum/pressure to drive the gyros. The standard regulator is adjustable to
set the input to the gyros to the required 4.5 - 5.5 on the gauge.

And yes, 6.75 is likely to shorten the life of the using equipment. The
gauge is marked that way because the instruments are designed for that level
of vacuum.

Dan Schaefer