[1865wingate]

When your washing machine empties it is usually under pressure from the pump, not just vented, and could pressurize your main vent, so you may not want the trap near the top of the washer drain pipe, This looks like it might be an older house before the 80's?

 

[1865wingate]

Do you have a problem with drainage every time it is used or only when multiple fixtures are draining at the same time?