Thanks guys, I ended up ordering a seal kit and have resealed my original box. Now I just need to swap it back in. I may open up the other box at some point and see if I can see the issue.

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1985 Mercedes 300D bought for $400 work in progress. Video Playlist: https://www.youtube.com/playlist?list=PLeYmvdiw3Xm87mFck3FjQ12ZoTSFXZ0Rr