Leaky Gearbox

I recently changed my clutch, so I had to remove everything from the bell housing back.

After I installed the clutch and put it all back together, I also changed the fluid in the gear box.
I used ATF.

Then my gear box started to leak !.

My mechanic has looked at it twice now and swears that it is leaking from some hose thingy that goes into the gearbox. He tried changing the copper washer but it still leaked.
He looked at it again today and still thinks it coming from this hose thing and wants me to get a replacement.

1. What would this hose thingy on the gearbox be?
2. Do you think that's my problem?

It never leaked before.

I am thinking it is one of two things.
1. When the tranny was dropped for the clutch change perhaps we hit/moved something (hose thingy) that's causing it to leak or,
2. When I changed the fluid for ATF the new fluid was so thin that it found ways to leak.
Keep in mind the old fluid that came out looked pretty bad, thick and brown and smelly.

1998 300tdi

thanks.

Gearbox braver pipe possibly

as above box breather , its probably cracked during removal, free end needs to be high as oil is splashed into the breather but should run back in to box ,not easy to get to banjo bolt as its close to diff lock linkage pivot
google frc9430,washers are 232039

Thanks !

So I went to order the parts and the very helpful person told me that before I spend the money try something very simple.
He said take out the fill plug and make sure that the gear box is not over filled as that can cause fluid to leak out of the breather hose.
I am going to try that this evening, would be lovely if that is the fix!

it would have to be full to the top still allways worth checking level , the gears throw oil up to the breather hole so unless secure it would still leak