Jump to content

Transmission leak, cannot find where its coming from!!


Recommended Posts

1970 Monte


Can anyone help me identify what this part is called? I have a small constant drip, empties out a quart in about 3-4 days when parked. Replaced the gasket, added silicone and changed the seals, and it doesn't seem to be leaking from the Torque Converter. We watched it drip while on the lift and it seems like its coming from somewhere inside this part. Any tips or suggestions would be greatly appreciated!!!




Link to comment
Share on other sites

That is the vacuum modulator valve. The gold small-body valve is an aftermarket replacement. The modulator has an internal rubber diaphragm that operates the valve in response to engine manifold vacuum. There is also an external o-ring that seals the modulator valve in the bore in the case. It's likely the o-ring is leaking. It is less likely that the rubber diaphragm or metal housing is damaged. Replacements are $15-20 everywhere.



  • Like 4
Link to comment
Share on other sites

As mentioned previously, the part you circled is the vacuum modulator.


I have also had a THM400 leak at one of the cooling line adapters, above the modulator. Check high on the case to see if the leak originates further up on the transmission.


The shifter shaft is on the other side of the transmission.

  • Like 1
Link to comment
Share on other sites

16 hours ago, Billywalshbigblock said:

Replaced the O ring already when we checked seals.


We actually only changed the real seal and pinion, is it possible it can be coming from the Shifter seal??


The leak can be almost anything. Modulator, cooling line fittings, fill tube, pan gasket, tailhousing gasket, pressure test plug, porous case, etc, etc. The shifter shaft seal and speedo drive housing are on the driver's side of the trans, however.

Edited by joe_padavano (see edit history)
Link to comment
Share on other sites

  • 2 months later...

FWIW, I had a similar leak on my '67 Riviera (ST 400 transmission).  Changed pan gasket and output shaft seal (pan and gasket was wet).  Still leaked and couldn't figure out where the leak was coming from.  Took a closer look at the dipstick tube and shift shaft area and just happened to notice that the vacuum modulator retainer looked a little funny (i.e., 'bent' rather than straight).  Removed the bolt and it fell out in 2 pieces - broken in two just past the bolt hole.  Temporarily reinstalled with a piece of flat steel reinforcement and large flat washer - leak seems to be gone.  Ordered a new retainer which I hope to install this weekend. 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...