Leaking trans .. want to replace seals

Last fall when I got "Hulk" I did a transh fluid "flush" pulled pan, new gasket and filter and new fluid, pumped the new ATF put the trans cooler twice to flush out as much as I could. Filled it up and good to go.
I'm know the myth of new fluid will cause leaks... Not interested in discussing that topic or the fluid I used which was Valvoline MAX life ATF.. I also flushed the Power steering with it as well. I use it in all my cars that call for it as was sure that the Skylark was fine with it, I looked it up here.
Now she leaks bad from the side round cover. I'm a total newb when it comes to transmissions but I'm a very skilled home brew mechanic. I'm sure I can do it but would like some advice on replacing the seals on the trans.
Should I DIY this or send it out, if so to whom in Denver CO area?
What can I do to the stock trans to (upgrade) it. I'm not going crazy with power. Down the road I plan on doing a 4 speed manual swap. I might just pull that side cover and replace the gaskets as that seems to be the worst leak. But the input shaft seal was leaking when I got her.

 

Sounds like stage 1 of a frame off to me…..

I would do the side cover seal and let it ride if it was me..

If you’re feeling frisky….. You could drop the transmission and do the F&R seals along
With the U Joints and check the pinion seal to make sure it’s dry. A good day’s worth of work.

And I would hope you find the rear main and oil pan nice and dry, just needing a good clean up and reassemble….

 

I have had really good luck with Grant at Denver Transmission.

 

I take it that it is a TH350 trans? If so, the round cover is an accumulator cover. It has 1 o-ring to seal it. It has a retaining clip holding the cover in. You can use a small punch through a hole already in the case to move the retainer clip outward enough to pry it out with a screw driver. The cover fits tightly but will pry out.

Reassembly should be easy to figure out. It might be time well spent to go ahead and replace the dipstick tube, kick down cable and shift shaft seals while you are at it. Fluid loss can be minimized with only raising the side of the car that your working on. Seals can probably be bought separately and less expensively from a local trans guy. Or you can get a gasket & seal kit from NAPA, etc. that should have all you need in it.

 

Chased a leak coming from the right-side of a TH350. Replaced seals all over and finally figured out it was coming from the Trans spark control switch. Replaced that and no more leak.

 

good advice... By any chance, do you know what the OD of the oring is... i have a BIG drawer full of oring materials, i might have what i need on hand!

 

Sorry, no I don't. It is also fairly thick.

According to NAPAOnline.com, the accumulator seal for a TH350 measures 2.33 in in diameter and .139 in thickness......https://www.napaonline.com/en/p/ATP14879

 

2-228 or 2-229

 

https://www.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/ORD-5700.pdf

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