Technical Vacuum Modulator

I have a trans fluid leak from behind my TH400 vacuum modulator. I bought a new modulator from Autozone and I still have a slow drip. It seems to drip mostly after running at freeway speed. I dunno, maybe there is a scratch in the housing behind the seal? Anyone have this issue before?
Thanks,
Ken

 

The vent tube is above the modulator. If the transmission is overfilled, it will blow some oil out of the vent at highway speeds.

Also, the dipstick tube O ring can leak if the tube is bent slightly.

Look carefully, to make sure where the leak is. It can be difficult.

 

Thanks Squirrel, I did check the vent tube and it seems to be dry and clear. I will check the dip stick.

 

It is possible that there is a scratch at the O ring sealing surface in the transmission, but since it's a recess that the O ring can seal at the seat and around the outer edge, it would have to be a heck of a scratch.

 

If you had a TH350, then I would look at the accumulator cover o-ring, but you don't. Perhaps the 400's have something similar.

 

Cooler line fittings are above the modulator...no other stuff in the area, aside from the vent and filler tube

 

I recently installed a new Derale dual fan trans fluid cooler. The trans lines are braided and fittings are AN. The leak was there before I installed the cooler. I have an after market dip stick that has a questionable tip that goes into the trans, I need to replace that to ensure I'm not overfull.
Thanks for all the replies.

 

It's inside the pan.