# Transmission Leak



## Praxis (Feb 23, 2014)

Help!
1967 GTO with TH400 that was rebuilt a bit ago. I have a persistent leak of trans fluid that is sometimes worse than others. The trans has been rebuilt, added a deeper sump with drain plug, kept having small puddles of fluid.
Recently had lots of smoke during a drive so got it to a shop, on the lift numerous leak points identified including fluid from vent, all corrected (governor cover, speedo drive cover, new pan gasket, etc.) and all was better, but still small leak based on garage floor appearance. It is entirely possible that shop slightly overfilled the trans.
Brought it to a really great mechanic, up on his lift discovered the drain plug was loose by 1/2 turn, tightened everything, cleaned off trans, ran car in gear on lift up to 40 mph indicated and observed no leaks anywhere, drove 5 miles home and parked with new clean cardboard under car. Hours later, cardboard clean. Great!
Next morning puddle of at least an ounce on cardboard, looks like it mainly came off right front area of trans, drops of fluid still hanging off right front of pan. Cannot see further because exhaust blocks view, and no lift.
It’s going back to mechanic next week, but my question is: could leaks from the top vent tube happen that many hours later after driving and not at all during first few hours? If leaks are from cooling tubes, shouldn’t it have leaked while we were checking? If the leak is from the dipstick tube, would that explain the delay in the leak, fluid settles into sump after sitting?
Wish I had a lift.


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## Praxis (Feb 23, 2014)

Also, any recommendations for a correct fitting direct replacement trans dips stick tube? Where to buy? I have ordered a vent catch tank and will install.


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## michaelfind (Dec 14, 2018)

Praxis said:


> Help!
> 1967 GTO with TH400 that was rebuilt a bit ago. I have a persistent leak of trans fluid that is sometimes worse than others. The trans has been rebuilt, added a deeper sump with drain plug, kept having small puddles of fluid.
> Recently had lots of smoke during a drive so got it to a shop, on the lift numerous leak points identified including fluid from vent, all corrected (governor cover, speedo drive cover, new pan gasket, etc.) and all was better, but still small leak based on garage floor appearance. It is entirely possible that shop slightly overfilled the trans.
> Brought it to a really great mechanic, up on his lift discovered the drain plug was loose by 1/2 turn, tightened everything, cleaned off trans, ran car in gear on lift up to 40 mph indicated and observed no leaks anywhere, drove 5 miles home and parked with new clean cardboard under car. Hours later, cardboard clean. Great!
> ...


I'll be watching for responses too. I don't have a certain answer for you but have read in Cliff Ruggle's book that the o-rings on the dipstick tube tend to leak. That has certainly been true for me for many years over many TH400s. There is a multi seal plug to replace the o-ring but I've not yet installed one to be able to say it's great or junk. Perhaps this is something to look into.

I will ask, are you sure it is not overfull?


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## armyadarkness (Dec 7, 2020)

I have a 67 with the TH400, and many of the same issues.

Smoke during a drive is a bad modulator. Its a $12, 5 minute fix. Also, that oring can leak... My trans is also fresh, deep sump, all new sealed gaskets... still get the puddle! Anywho, I got my stick from Inline Tube.


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## michaelfind (Dec 14, 2018)

armyadarkness said:


> I have a 67 with the TH400, and many of the same issues.
> 
> Smoke during a drive is a bad modulator. Its a $12, 5 minute fix. Also, that oring can leak... My trans is also fresh, deep sump, all new sealed gaskets... still get the puddle! Anywho, I got my stick from Inline Tube.


I have been told by some "experts" that they all leak, always did, even when practically new. I've tried to prove them wrong over the years but still end up moving my little drip pan along with my GTO whenever we move. 😕


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## armyadarkness (Dec 7, 2020)

I can tell you, I used a B&M aluminum pan with Permatex Utra Grey, and I still have leaks. Which is nearly impossible.


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## Praxis (Feb 23, 2014)

michaelfind said:


> I'll be watching for responses too. I don't have a certain answer for you but have read in Cliff Ruggle's book that the o-rings on the dipstick tube tend to leak. That has certainly been true for me for many years over many TH400s. There is a multi seal plug to replace the o-ring but I've not yet installed one to be able to say it's great or junk. Perhaps this is something to look into.
> 
> I will ask, are you sure it is not overfull?


It may very well be overfill.


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## armyadarkness (Dec 7, 2020)

Praxis said:


> It may very well be overfill.


Not likely. At least mine is not.


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## Sick467 (Oct 29, 2019)

I have never had a vent tube leak unless it was overfull and then it was a puke and purge not an ongoing leak. What's going on in this scenario is the over full fluid is heating up and expanding such that it becomes over-over-full and the vent allows that excess to escape. If the system only got "so hot" on the first drive, if may not have puked all that it can and will do so the next time the transmission gets even hotter. So, if the first test did not allow the transmission to get up to higher temps of operation...A purge may not have happened, but a good street drive could prove to get up to that magic temp. 

A dipstick seal leak would be minor to non-existent while driving, but , as you said, once the fluid drains back to the pan it will become much more evident. The few transmissions I have had the pleasure to work on have all had the fill level above the pan gasket (TH 350's and 400's as well as the [email protected] AOD). This means that once all the fluid settles back to the pan...it's at or above the dipstick seal.

The right front leak sure points to a dipstick, vent, or modulator leak, but depending on how the transmission is leaning it could be hitting the pan to case split and migrating around to the lowest area and then dripping off.

One common place for the well used Th400's to leak is where the shift linkage goes through the case (Passenger side, just above the pan). Once the shaft has worn, the seal does little to hold back the fluid. This leak is typically minor, but will be enough to eventually run along the case and then make it's way across the pan gasket and end up dripping off the pan..."somewhere". This makes it hard to pinpoint and, at first glance...looks like a pan gasket leak.

My transmission had this problem and I fought with it for some time until I put a new shifter shaft and seal in. You can just put in a new seal without much trouble, which I did first, but if the shaft is ~50 years old...I'd just go the extra steps and replace the shaft too. Another cheat is to add a seal to the shaft on top of the other one. There's room for two in there, but that didn't work for me either.


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## michaelfind (Dec 14, 2018)

Sick467 said:


> I have never had a vent tube leak unless it was overfull and then it was a puke and purge not an ongoing leak. What's going on in this scenario is the over full fluid is heating up and expanding such that it becomes over-over-full and the vent allows that excess to escape. If the system only got "so hot" on the first drive, if may not have puked all that it can and will do so the next time the transmission gets even hotter. So, if the first test did not allow the transmission to get up to higher temps of operation...A purge may not have happened, but a good street drive could prove to get up to that magic temp.
> 
> A dipstick seal leak would be minor to non-existent while driving, but , as you said, once the fluid drains back to the pan it will become much more evident. The few transmissions I have had the pleasure to work on have all had the fill level above the pan gasket (TH 350's and 400's as well as the [email protected] AOD). This means that once all the fluid settles back to the pan...it's at or above the dipstick seal.
> 
> ...


Excellent description of what may be going on for many of us ! Thank you


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## Sick467 (Oct 29, 2019)

michaelfind said:


> Excellent description of what may be going on for many of us ! Thank you


No problem MF. Leaks are a bummer and often something we live with. Every one of my old V8's (4 total) drip a bit of this or that - sometimes all the time, other times today, but not tomorrow...lol.

Here's a trick when you have had about enough of trying to find where "IT" is coming from...

Clean the area well with degreaser, pressure sprayer, thinner, carb cleaner, or whatever makes you happy and the job easy. Let it dry really well and spray it with some foot powder spray. This will leave a white film of powder on the area and will help to show you where the leak is originating from. The leaking fluid will leave a trail in the powder film. This stuff cleans off real easy with compressed air, wiping off, and/or water. It is great tool for chasing down small leaks. The hard part comes after you find that it's the seal that means you have to work all weekend to change out...lol!

I'd like to also mention that the OP's leak being near the front right may also be the lines that go to the radiator at the transmission case.


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