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Finally found and fixed that %&$@ transmission leak!


EmTee
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I've been hunting for this transmission leak on my '67 Riviera for a year now.  Replaced the pan gasket because there was fluid around the outside of the gasket and thought that would solve it, but no...  Then assumed it might be the dipstick tube O-ring or maybe the shift shaft seal.  It seems to leak a pie-pan size puddle after the car sits for a week or more as the torque converter drains down.  A couple of weeks ago I decided I was tired of changing the cardboard on the floor and adding more fluid and got back under the car to try once more to see whether I could figure out the source.  As I was looking at the area where the dipstick tube meets the transmission case I noticed something 'funny' about the retainer that holds the vacuum modulator in place.  What should be a straight steel bar looked like it had a bend, or crease just past the bolt hole.  I removed the retaining bolt and the piece between the 'bend' and the modulator fell on the floor while the piece with the hole was with the bolt in my hand.

 

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I then removed the modulator and about a quart of ATF came pouring out.  The O-ring on the modulator was intact, so I reassembled the modulator and temporarily backed the broken retainer with a piece of flat steel just to see if it would stop the leak.  After a test drive and check the next day it looked good, so I looked online and found a replacement retainer on Ebay (https://www.classictransmissionsolutions.com/th400-vacuum-modulator-retainer-transmission-clip-hold-down-bracket-turbo-400/)

 

Here's the final installation.  I decided to include a flat washer under the bolt head just to spread the load.  Hopefully I can put the kitty litter away for awhile now...

 

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Even with a new o-ring the dipstick tube can leak with the same timing and "evidence" characteristics as the OP's case.  A fix is to wrap oil-resistant pipe tape under the o-ring to increase its OD while providing a better ID seal.  Only wrap around ~ 2-3 times else it will be too difficult to reinsert.

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