Hi Mark and Welcome to the Forum! 
There are a few ways to go about this. The first way I describe you go to the input routes of your songs that target your racks and open the route configuration box and check the “Notes Only (suppress all other events)”
but when you do you need to use bindings (as I assume you did to set up your rack gain sliders) to map the controllers you need from the keyboard for each rack (like sustain pedal, expression, etc…).
The next way you can try uses MIDI route filters to do it using the suppress events filter on each input route. This allows you to block all but the CC messages you want to pass to the rack(s). In this example I blocked all CCs except the expression, damper and sostenuto. The bindings you made to control the gain on the racks are unaffected by these filters.

Those are some options for you, I hope they help.
Cheers,
Dave