Hi Mylkö,
go to P'08 global settings and set
- MIDI Control to OFF and
- MIDI Program to ON.
The first command will make P'08 neither send nor receive any MIDI controls, which will solve Your volume-problem, but also hinders pitch and mod wheels, pedal, and breath controller from transceiving MIDI-CCs. See P'08 manual page 40 for reference.
Second command will keep program change messages coming through (remember: in both directions, so program changes dialled in on P'08 will be sent to Your pedal, too)
If You need those other controllers or do not want program changes to become effective in both directions, You will have to dig deeper into MIDI and look into Your pedal's and the P'08 MIDI implementation.
Using NRPNs instead of CCs might solve problems, special cases may even require MIDI filtering. But give the method described above the first try.
HTH
Martin