Replacing air hoses under the seat

Are you talking about air line coming through the floor carpet towards seat or about seat air lines?

If this standard Freightliner’s ISRI seat, move seat cushion forward to the maximum (only cushion, not the whole seat) and move it up while doing that. This way you will remove it and will have access to the all air lines under it.
If your seat is International - you have to move it’s seating cushion also forward and pull it from the buck of the cushion.

 

What truck is it? Volvos have National Seat brand seats, you would need to figure out where it’s leaking and try to source replacement connectors, switches. It’s easy but you may end up needing to delete some adjustments to minimize number of connections down there if you don’t have parts - they are hard to come by. I just left height adjustment and lumbar, the rest is deleted for now.