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Any Ideas on how to stop a 727 Transmission from leaking fluid around the fill tube?

Paul Cotton

Well-Known Member
Local time
2:14 PM
Jul 19, 2016
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Marion, Ohio
I have a 727 transmission behind a 440 in my 1965 Plymouth Belvedere II. It is leaking around the dip stick fill tube, where the tube plugs into the transmission. Actually, leak isn't a good description of the problem. It is squirting/spewing/spraying out and onto the headers, causing quite a mess as well as smoke and smell.

Last fall I had the same problem and replaced the o-ring with a new one, then placed a little sealer around the o-ring as I installed it. It seemed to take care of the problem, but now it's leaking again.

Before I put my car up on jack stands and crawl under it to address this again, I was wondering if anyone has had a similar problem and how you addressed it.

Any advice/tips on how to stop this leak permanently will be appreciated.
Well first make sure you have the correct dipstick length and tube length for a big block 727.
Then make sure you haven't overfilled the pan capacity.
Third, replace the O ring anyway, but clean the dipstick tube and the tube well in the trans, with solvent so it makes a nice non slippery fluid free seal.
Problem should be solved.
I found a slightly larger OD" O" ring at the Hardware store.
Be sure tube is perfectly round.
A small bead of silicone near the top if desperate but not where
it can get in trans.
If it is squirting or spraying out then you have another problem Besides that oring.

Take a paper towel and wrap it around the tube at the trans case, now tape it in place and test to see if the leak is contained inside the towel.

maybe look at the pressure test plus in the side of the case for the 2nd gear band, accumulator and low reverse band. Make sure they aren’t leaking they could spray.

If that oring is sealed it won’t leak no matter if it is overfillEd, it’s always got fluid there anyway.
Check the things as the others have stated. I've always put a dab of RTV/silicone around the o ring and never had an issue with the tube itself leaking.
All good answers and, take a good look it isn't cracked.
I use the white permatex pipe thread sealant.( pipe dope)
Is the tube bolted to the engine so it won't move, vibrate and break the seal?
I would suggest draining a couple quarts before you work on it.
An easy method is to remove the rear trans cooler line and put a hose on it.
Start the car in Neutral for long enough to pump out a couple quarts.
Then you can remove the tube and clean any residual fluid from the opening as well as the tube and seal it when you re-assemble.
I've never had to use any kind of sealant.

However, that O ring is a common problem area. Usually once and done, though.

1 year is WAY too short of a lifespan, unless it was a junk part to begin with.

That said, rubber "ain't what it used to be".
I've used the dipstick linked in post #10, they're great if you don't care about originality. They use a grommet which is far superior.
If you want to keep it original, check your tube for cracks, I've seen more than one tube cracked in the area around the o-ring.
Squirting and spewing is concerning, sounds like you could have bigger problems? A plugged vent can certainly cause a lot of problems but that should only become an issue once the transmission is good and warm, if it's leaking even when it's cold I wouldn't suspect that to be the case. Unfortunately I believe the vent is behind the torque converter so no easy way to check that.

If you have a FSM they show you how to test the different circuits with air pressure, that would at least let know if you have an internal leak. Then as suggested test the psi as well. Hope it's just the o ring but I've never seen one spew or squirt unless there's pressure present which there shouldn't be.