hi poniu,
You can have a look on this
tutorial who make what you want.
In substance controller are drived by a value included in a range of [0..1].
If you invert your presssure you will got [-1..0] who will be clamped to 0 and generated no effect at all, so you need to Add 1 to your result to retrieve the valid range [0..1]
In your case :
ControlledValue = 1 - PressureThis can be directly achieved by the Sub operator when you plug your pressure on
Input B and set 1 in
input AHope this help