As someone said you could fill the bridge, cap it, submerge it, then uncap it.
Or you could submerge the bridge, insert a hose and suck out the air.
A different way to suck out the air would take more work when setting up the system, but would help in the long run. Drill a hole at the highest point of the bridge. Connect a check valve to the hole so that you could pull air out, but none would flow in.
Does that make sense?