Sorry to take so long to get back to this but with the holidays, a graduation and the usual non-organ things happening around here it's been hectic. So I studied the schematics that I've got for the Trio's but I'm not immediately seeing a failure that could cause the symptoms you're decribing. On my schematics I'm seeing the pedal voices being mixed into both the main and tibia channels just before the expression control circuits. They're then fed to the headphone and power amplifiers as well as the reverb which gets mixed back into both channels. So it's not obvious how the pedal signal could be affected without causing some disturbance in the main or tibia outputs.
Let me restate what I think you're seeing just to make sure I've got the right picture a I'll dig a bit more into it.
1. When the reverb is turned all the way up the pedal, main and tibia voices all sound OK (except for the excess reverb)?
2. If you turn down the reverb then the main and tibia voices are still OK but the pedal drops way down?
3. Are you seeing this problem both while using the headphones and when you're using the speakers?
4. Do you know specifically what model of Trio that you have? 321, 321A, 321B, 321C (If not, can you provide the serial number.)
I have noted some minor differences between the way my Trio's are built and what's shown in the schematics so maybe we're running into that issue. I had to manually trace out some of the circuits to fix a problem or two on mine. I've heard that there were "improvements" in the different model runs so a particular set of diagrams might not match all of the instruments in a series. Work is shutting down for two weeks over the holidays so I should have more time to spend on my organ hobbies :^)