Have a 78 Vette with a leaky TH350. Have tried everything I can think of to find it. Searched all the forums and did a big internet search and cannot find the problem. Trans leaks about a puddle about 4 inches in diameter when parked after running. It looks like it is coming out of the pan or the pan drain plug (has an aluminum pan). Cleaned the entire trans with brake cleaner and dusted it with baby powder to try and find the leak. Checked the filler tube, shift shaft, speedo plug, kick down port, vacuum modulator, and governor housing. All good no leaks. Three of the pan bolts on the passenger side show a slight weeping but not enough to cause the 4 inch drip spot. Any ideas??

 

Mine turned out to be the LARGE O-Ring for the internal Pump...it was rock hard and not sealing anymore.
I pulled the TH350c myself in a 9' wide garage (that sucked) and ended up rebuilding the entire Transmission myself.
Actually it was a good thing as one of the Planetary Gears was self destructing.

 

Front seal? If so, it would run down the front of the TH350 and drip near the front of the pan onto the floor... You would still need to pull to fix that. There is an inexpensive "hook" hand tool you can buy to pry the seal out (around the front input shaft) and then you install a new one (grease it and tap in with a plastic hammer -- it is a press fit round seal about 2.5" in diameter). It seals around the shaft of the torque converter.

 

I'm talking front seal-large O RIng...and mine would wick to the side of the tranny...I checked the Pan Gasket, the shifter arm exits, you name it. Try removing the Flywheel cover-guard, that helped me to spot it.

 

I agree it is probably the large pump gasket that Hammerdown mentioned about. I had that leak on my 78 and had the transmission rebuilt. The pump probably needs a rebuild/replacement so
just changing the seal may cause other issues. Just rebuild the transmission.

 

I would be inclined to believe the leak is from your torque converter seal.
pull the inspection cover. If it is wet inside the bellhousing, its most likely the tc seal. Not a difficult repair, if you don't want to spend a lot of dough on the correct seal driver, you can make one out of heavy wall PVC or abs pipe. Biggest pain is getting the trans out.
I would also recommend a service manual. Stay away from the Haynes/ climes types, get a factory manual from ebay

 

 

While I like Greengear's suggestion "just keep adding fluid!" have to agree with others that a good rebuild is probably the prudent course of action.

My 78 THM has been leaking for probably fifteen years and I've traced it to, what I believe to be the torque converter seal, which means pulling the transmission. I had other leaks as well and had a local shop do a flush and gasket replacement which only corrected the pan leaks. Is the front seal hammer describes above same as the torque converter seal?
If you're pulling the THM in your backyard garage shop, do you use a tranny lift to ease it out and down? Sounds like that is the greatest challenge if I'm doing this myself.

 

Transmission was rebuilt about 5k miles ago. When the engine was replaced about a couple months I replaced the converter seal as a prudent measure. Will pull the converter cover and see if it is leaking inside there. If it is leaking I can't decide whether it is easier to pull the trans or the motor. It is a restomod so there isn't all the extra stuff hanging off the engine so the engine is pretty easy and there is no good way to pull the trans without a lift. Then again like Greengear said I may just keep adding fluid.

 

 

If it were me, I'd drop the trans. Run a bar (2x4, angle iron from a bed rail, piece of unistrut) across the fender/hood lip at the back of the motor and use a strap to hold the back of the motor up, put the car on jack stands, disconnect the driveshaft, speedo cable, shift cable, and cooler lines, pull the filler tube, unbolt the tc from the flexplate, unbolt the trans from the cross member, drop the cross member, unbolt the trans from the engine and lower it. Might want to drain the pan and save a mess when you pull the fill/ dipstick tube.
Piece of cake. I think harbor freight sells a trans adapter that you put on a floor jack. Or you can do it the way I've always done it and bench press the damn thing, just be careful the tc doesn't slide out and hit the floor. Been there, done that.
be sure to check for a groove cut into the tc snout, cause if it is bad enough, no seal will fix it.
Only other thing I remember causing issue is when you put the tc back into the trans, rotate it and you will feel it engage 3 times as it turns and each time it will clunk in a little bit further, when it is all the way in, you won't be able to get your fingers between it and the bell housing.
If you find that it is the pump seal, you will need a slide hammer to pull the pump. That's not a huge job either, but I would again, recommend that you get a good factory service manual.
I have a tear down and reassembly manual floating around here somewhere from when I went to school for this stuff, I'll see if I can find it and post a few pics.

 

But then again it maybe easier to pop the motor out if you don't have PS, ac, and a bunch of pollution controls to deal with. Since the motor was recently installed, the exhaust should unbolt easily.
About 6 of one and half a dozen of the other. Lol