TH400 Tranny Spurts Out of Vent

..."Overfull, overheating, loose/cracked/damaged filter extension up near case entry, loose valve body bolts/leaks at valve body, or warped/mismatched pump halves are all potential causes for fluid venting."...

"I run a Rossler built 400 in the truck. It wasn't over filled, but about every 5 passes it would have may 2 ounces of fluid in it the overflow. I was told this was normal. 4110 lb truck @ 8.99 153.5 mph in the 1.4."

...."Sometimes they push oil. Many "builders" don't seem to have the ability or be willing to take the time to make a pump surface / stator half flat and many times the factory finish is not up to par with the pressures that are being forced through these today."...

https://www.yellowbullet.com/forum/showthread.php?t=1761962

Assuming you're not racing this at 150mph, my guess is an aluminum casing crack, allowing pressurized fluid direct access to a drain passage, passage not designed for any sustained return flow. Never ran across this in a GM Turbo 400, it's a venerable beast. Depending on age, a small inclusion in the casting can take years to finally turn into a hole. You'd think that the loss of pressure in the clutch pack or band servo would make the vehicle undriveable, unless maybe you might not notice loss of torque (slipping) in Reverse?

No racing, just cruising in my T bucket. But I've had it for 25 years and history before that is unknown. I'm ready to just swap in a newly rebuilt unit.

That’s a great run, she doesn’t owe you much!

The hydraulic pump may be sucking air under certain conditions causing foaming and overflow.