technical question for RC gurus

[andyathome]

ok, I have a rc4wd excavator .
I have fitted a truck sound modual to the excavator.
I am using the left track as the motor start signal and am using the 4th valve (hydraulic bypass)as the speed reference for the sound card.

problem is :that obviously when you activate the arms , bucket ect the motor sound slows because the valve closes to allow oil flow to increase.
what I want to happen is to have the engine sound increase when I use the hydraulics.

is there any way to reverse the signal that comes out of the reciver.
I have made 2 y leads and attached to the reciver as per instructions, so if there is something I can plug in that wont affect the operation of the 4th valve that would solve all my problems.

cheers for your help.
andy