TH 400 Transmission Leak When Parked

[72step]

Last year I bought a seal and gasket kit for my tranny because it was leaking on my garage floor. I replaced every seal I could without taking the tranny off and rebuilding. The tail housing seal, dipstick seal, tranny pan, speedo seal etc... It remedied the leak for about a year until it suddenly leaked fluid onto my floor again. Apparently it's not leaking too much because I bet I could go out there and put a quart or 2 and it would run fine.
The thing is when I put fluid and go for a drive is the tranny works like the day I got it rebuilt. It has a TPI racing kit in it so it runs great.
Now if I take it out and in to a shop, would I be best just doing a complete rebuild? Or could it be a certain seal? I guess my best bet is to buy a complete Performance rebuild kit and take the tranny in to a shop. This is getting old, because this is will be the second time I have to rebuild it, as it started life out as a TCI racing tranny I bought from Jegs some 11 years ago, then around 1999 I had to have it rebuilt because it was shuddering.
Are TH400's known to do this?

[Bus Ted Knuckle]

Did you replace the selector shaft seal? I did not see it on your list, and it can leak like a bugger. You might want to re-torque the pan bolts too, sometimes they need a little snug up.

[67chevyguy]

my plan of attack would be to wipe down the tranny with a rag, get it nice and clean. Then park it for the night, go check it tomorrow and see if I could pinpoint where it's leaking from. No need to get hasty and take it in to a shop when it could just be a slow leak from one of the seals that you can change out in an hour.

When you changed the seals did you change the speedo gear? after a while they can get worn down just slightly enough that they will let fluid seep out.

my 2 cents

[Bobcherry13]

I saw this problem on another site and I had it with my 69 with a 400. There is some sort of pressure release at the rear of the transmission (forgive me on terms) where the drive shaft bolts up. My leak stopped once I drove it frequently. If it sat the pressure or vacume would release and a big pool appeared.

[72step]

Thanks.
I actually did change out the speedo gear as well. I don't remember for sure about the shifter seal, but I'm pretty sure I did as I purchased the entire gasket kit for a 400. I actually posted about the issue here before I did the work and I did as everyone on here advised. Also when I did do the job, I couldn't for the world see where it was leaking from. I know part of the problem is that I don't use the truck enough and the fluid tends to leak when it seeps down into the tranny pan. I guess I can try and get underneath again to see where it leaks but it's hard to see because tranny fluid is so thin and just kinds runs down.
I wonder if it couldn't be the front seal where the torque converter mates?

[72step]

BobCherry13, Well see, that's the problem, if I drove it around at least once a week, it wouldn't be an issue. Usually if I drive it around then park it; it will then take 3-4 weeks to start leaking again. Problem is that I just don't have the time to drive it.--Or the money being that gas is where it is and the truck runs on super-unleaded and does about 10 mpg or less! Especially being that I don't exactly baby it when I do drive it as it has a high performance 327.

All things considered, I'd be better off installing a newer fuel injected engine in there such as a 5.3 with tranny and all but that again runs into more money.

[Green Machine]

Sounds like convertor drain back to me. Check the dipstick after it has been shut off a few minutes and then check it a week later, engine off again. Bet you will find the tranny is way overfilled. Could be a bad convertor or loose bushings around the input shaft.

The convertor holds around 6 quarts of fluid. If it drains down to the center hub, your tranny will be 3 quarts overfilled. Even new seals will not hold that fluid back. Check and see if that is what is going on. If that is what it is, it does not hurt anything, but can be a pain to live with. Might consider a rebuild in the future, but tell them what is going on so they can check for drainback fixes.

[kelsfine69]

My TH400 aways leakes around the GM tin pan. Switched to a cast aluminum with a nice gasket(rubber) and problem solved

[Lugnut64052]

Originally Posted by Green Machine

Sounds like convertor drain back to me. Check the dipstick after it has been shut off a few minutes and then check it a week later, engine off again. Bet you will find the tranny is way overfilled. Could be a bad convertor or loose bushings around the input shaft.

[jayharrell]

I found you need to make sure the overflow tube on top of the trany needs to be clear and open. If its plugged up or even removed and pluged with bolt, the pressure will cause it to leak out the rear seal.