PC 88 Soft Pedal - Can't seem to control soft volume

I appear to be setting up a soft pedal on a PC88, I do have control and am able to hear a slight volume reduction either with toggle or momentary pedal action. But, I can't seem to set a soft volume level. I have tried using 0 / 127 and 127 / 0 for on / off settings with no discernable volume results. Any hints on controlling soft volume?

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