TH400 vacuum modulator question

[BigT]

The vacuum modulator on the side of my transmission appears to be leaking slightly, probably just a bad O-ring. What does it do, and how would I know if it needs to be replaced?

 

[Sad Sack]

How to Test a TH400 Vacuum Modulator

The TH400 is an early model, non-computer-operated transmission widely sought after by car enthusiasts for its internal strength. The fact that it is not electrically actuated by solenoids means it requires other apparatus to regulate shift points. Simply put, an automatic transmission, whether...

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[YakkoWarner]

I replaced mine a couple years ago - definately had fluid coming out around it. Its easy to do, but have a catch pan handy because you will have a gusher of fluid coming out.

 

[Ricko1966]

The vacuum modulator controls shift points,and transmission line pressure. Pull the vacuum hose off it,if there is fluid in the hose replace the modulator. If your shifts seem timed right and correct firmness,not too lazy,not too firm and there is no fluid in the vacuum side if the modulator I would not replace it. The mentality of its old just replace it,isn't a good policy anymore a lot of new parts fail in a short time. The quality just isn't there anymore.