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Assuming your leak is at the pan-body interface, this is what cured my leaky pan once and for all:

1. Pan reinforcing plate from Bronco Graveyard.
https://shop.broncograveyard.com/C4-Pan-Reinforcement/productinfo/11105/
I guess the Broncos have a a slightly different pan design as I had to notch the inside of one corner of the plate with a Dremel to fit the one corner of the Mustang pan that's rounded and then it fit perfectly.

2. Permatex's Ultra Gray like GypsyR mentioned. I removed the pan and let the fluid drip for a day or two in order to get the bottom surface of the body as dry as possible. I used a thin coat of the Permatex on both sides of the gasket.

3. Farpak fiber gasket. My trans shop used this gasket on a rebuild and it lasted more than 4 years. A bought several from them after that. You can try a local trans shop or I see them on eBay too.

When I tried to replace the gasket that leaked after four years, I couldn't get it to seal as good as the trans shop until I got the reinforcing plate from Bronco Graveyard. That was 16 months ago and it's been bone dry ever since! I think what the plate does is it spreads the load of the mounting bolts over the entire circumference of the gasket rather than just at the bolt holes providing for a solid seal all around the gasket and little or no distortion of the pan flange at the bolt holes.

Hope that helps.
Bill
 

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saw one that appeared to be leaking at the pan/case but was actually coming from the vent tube.
apparently it had been slightly over filled. possible it may have had the wrong dip stick?
 

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Discussion Starter #23
You really took a higher road than I expected. I'm sure a flycut off the edge is the correct way to go. While you have it there ask if they can check the case for cracks. Just because. Actually you or anybody can because it's just a matter of investing in a two spray can dye check kit from someone like Magnaflux. You don't need the equipment for checking like you do for cast iron, just the spray dyes.

If you haven't already, run the case through a glass bead or walnut shell blasting. It makes the case look beautiful and brand new.

For you folks with time and more than a small amount of patience you can accomplish the same thing with something called "hand scraping". Usually done on iron and steel, the same technique works on aluminum. And is a heck of lot easier because the subject material is so much softer. Properly done you can "flatten" a surface to be more correct than any machine your local machine has in their arsenal. People have been using hand scraping for well over a hundred years on stuff like lathe beds where it's absolutely critical they be flat and straight. Google if you're interested.

I mention this because I have the ability to flycut things and can imagine a shop being a bit less than enthusiastic about mounting a transmission case in one of their machines. It's doable, just a bit out of the ordinary.

Yeah, pretty much figure it was just the right thing to do and I will not regret it down the road. Last time I had it out I did a full restoration of it, so it's been blasted and cleaned spotless and rebuilt it and added all the original marks I found on it, runs and looks great, but just basically blew it by not doing a flycut off as you mention. Also, very good idea to have them check it for cracks as well...

I watched a few videos on hand scraping, now that looks somewhat like an art form, seem like you can do it and probably get good at it after sometime... but really cool and never knew about this...

Thanks as always,

Jason
 

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1. Pan reinforcing plate from Bronco Graveyard.
https://shop.broncograveyard.com/C4-Pan-Reinforcement/productinfo/11105/
I guess the Broncos have a a slightly different pan design as I had to notch the inside of one corner of the plate with a Dremel to fit the one corner of the Mustang pan that's rounded and then it fit perfectly.
I did this too, just be careful not to strip pan screws; they're barely long enough with the reinforcement plate under them.

Any chance it's actually leaking from the front pump o-ring and gasket between the pump housing and the transmission housing? I had a stubborn pan gasket/fill tube leak that turned out to be the front pump o-ring. The rubber had shrunk and was no longer sealing. It's really hard to tell because the fluid seeps almost invisibly down from between the bellhousing and the pan flange and accumulates in the pan gasket seam, making it look like it's the pan or fill tube leaking. I had to spend half an hour under there with paper towels and a light after driving the car to warm up the fluid and fill the torque converter. I wish I had done that before replacing all the other seals in the transmission and still having it bleed out on the floor. It always seemed to leak a lot in the few hours after I drove it, and then just seep.

After replacing the front pump gasket and o-ring and input shaft seal, no leaks!
 

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Discussion Starter #25
This is my favorite picture of my car right now... after deciding to bite the bullet and pull the transmission out of the car again, tear it down to the case again and have the case pan edge machined, put it all back together and reinstalled in the car, been a week and no spots or leaks showing anywhere on the transmission. As I mentioned I wish I would have done this the first time I pulled it and rebuilt it (stupid brain). Also used a proper no cut out cork gasket with no RTV and an original pan, sealed right up first try.

So I guess the lesson is a powder check really does show you what you want to know, with my leak when powder checked it was leaking between the case and gasket, no where else, so the only true fix I guess was to mill it flat and remove the gouges. Also I still had some small pits after it was machined which I took a tooth pic and put JB weld in the pits, waited for it to set and then cut it with a razor blade and cleaned it flat (thanks to a suggestion by my machine shop).

So I guess and hope that's it for a while with this leaky transmission.

Thanks again everyone,

Jason
 

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This is my favorite picture of my car right now... after deciding to bite the bullet and pull the transmission out of the car again, tear it down to the case again and have the case pan edge machined, put it all back together and reinstalled in the car, been a week and no spots or leaks showing anywhere on the transmission. As I mentioned I wish I would have done this the first time I pulled it and rebuilt it (stupid brain). Also used a proper no cut out cork gasket with no RTV and an original pan, sealed right up first try.

So I guess the lesson is a powder check really does show you what you want to know, with my leak when powder checked it was leaking between the case and gasket, no where else, so the only true fix I guess was to mill it flat and remove the gouges. Also I still had some small pits after it was machined which I took a tooth pic and put JB weld in the pits, waited for it to set and then cut it with a razor blade and cleaned it flat (thanks to a suggestion by my machine shop).

So I guess and hope that's it for a while with this leaky transmission.

Thanks again everyone,

Jason
Nothing beats the satisfaction of beating an elusive problem!
 
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