Dipstick and vacuum modulator o-ring dimensions

[ChevDog1]

Looking for a bit of backup info before I head to the parts store today. I have a small transmission leak that is either the dipstick oring, or the oring on the vacuum modulator. The trans is a TH400. Does anyone know the correct sizes for these orings? My faith in auto parts stores is not what it once was. I used to have a parts guy at the local NAPA that could do this stuff from memory. Now they just look it up in a computer, and if you have a swapped in trans like I do, they can't help. Took me 20 minutes at the counter at AutoZone yesterday to get an ouotput shaft seal - I had to tell the kid that my truck was a 1978 C20 for him to find the correct part. Thanks!

[clapage6]

For the dipstick tube, those boogers always leak in my experience, I've only ever owned th400's with the o-ring but I like to clean off the stock o-ring and tube, then apply a bit of silicone to the tube and stab it back in there, snug it down, then wipe away any excess. Never had a problem...

The vac modulator is another story, might try taking that one off and either measuring it or bring it with you to find an appropriate match. A replacement modulator can be had real cheap will usually come with a new o-ring (might verify that before rodering.buying though)

[chengny]

I can't tell you the size of the o-ring (consider buying a whole kit for $10 at HF and trial and error it).

But it has to be of sufficient cross section that the neck of the modulator kind of snaps into place. If the oring is loose, when everything heats up it will leak. That clamp does a crappy job of keeping a tight seal.

[ChevDog1]

I stopped at o'reillys on the way home tonight. Had them pull a dipstick tube and modulator, then matched the orings form their bulk container to those.

I coated the dipstick oring with silicone before installing it. Also tweaked the modulator clamp to put mor pressure on it. I will top off the trans in the next couple of nights and start watching for leaks again...

[geezer#99]

You fix it with silicone guys kill me!! LOL!!
Why don't you fix the pressure problem on the tranny instead.
They do have a vent you know!!

[ChevDog1]

Mine isn't a pressure problem. It leaks continuously while parked. It would continue to drain until the fluid level was below the pan gasket when I started working on it. And the vent is clear - I checked that while I was under there. I just added the silicone for additional insurance, as mine has an el-cheapo chrome dipstick tube.

[Jonboy]

For the dipstick, upgrade to a newer design dipstick tube with the boot type seal. Stick, tube, and seal should be around $30 at the dealership. Probably save that in trans fluid pretty quickly.

[ChevDog1]

Good Idea - I'll keep that in mind if this doesn't work!

[Jonboy]

Once the tube gets slightly bent, it is almost impossible to get the O ring to seal. I chased it forever with a new trans build on my '75 Blazer years ago. replaced the tube, and the leak was gone.

To find it (if you aren't sure what is leaking), it was suggested to me to clean everything really well with brake cleaner, then spray some of the athlete's foot spray that turns white in the area you suspect. The red stands out on the white.