Your attached image is sideways, but am assuming the photo was shot with the back in portrait mode.
The leak appears at the bottom of the photo, so, that means the leak is at the top of your back.

When you take photos with the back oriented in landscape mode, is the leak always on the right side of image?
If so, that means the leak is due to the back, not the camera. Your leak is probably at the hinge or hinge end.

Is the back in question a 'RZ Pro' film back, or a 'RZ Pro II' film back?
You can change the seals in the RZ Pro film backs. The RZ Pro II's use a light trap configuration...no seals.

My two cents,
Marc