When I bought the car it had a transmission leak on the exhaust.  The pan gasket had postal strings between five of the pan bolts and I assumed someone held the rubber gasket to the trans and never took them out.  I repaced gasket and filter and had no leaks.  I started it today and had trans fluid blow out of the trans dipstick on high idle.  The car did sit for 10+years.  Any ideas? 

Couple o ideas...
Is the trans overfull? Check it before starting it next time. It should only be above the "Full Hot", or upper line about an inch. Then start it, and see how far down it goes. With the fluid hot(operating temp), the level should be no more than halfway up into the cross-hatch area, or in between the low & high marks. Trans fluid expands when it gets hot, and 400s tend to run a bit hotter than others...
One thing that could be happening is the torque converter draining back into the pan after sitting, as in overnight. This is a major problem with the 400. It the fluid level is too high to start with, and the converter drains back overnight, then there is waaaay too much fluid in the pan on start-up, and they can "burp" like that.
Another thing it could be is the vent tube for the trans is stopped up. You'd be surprised at the bugs that can crawl up in the tube and make nests, blocking any air flow thru the tube, especially if the car sat for a long time like that. The vent tube is on the top of the trans, and is usually a plastic tube that extends from the area above where the front edge of the pan is. Some have metal tubes, but most are plastic. With the fluid level at the correct level, you can use an air blower, and blow gently(15-20psi) down thru the filler tube to determine if the vent is stopped up. BE warned, tho...if the tube is stopped up, blowing air into the trans can cause the fluid to spit back out at you IF the air doesn't clear the blockage from the vent.

There could also be a problem with the fluid pump. When they warp, it can do the exact same thing...

Good call on this one Joel..I actually have nothing to add..!
Rich

Thank you for the quick response.  When I bought the car it had sat for long periods and the fluid did settle back into the pan.  A local shopkeeper and collector of Vettes told me to drive it and the seals might "soften" back up and keep the fluid in the converter and that seemed to work until it sat for over a month.  I will check the breather line also but suspect the trans needs to pulled in the spring. 

I don't think the trans would need a rebuild ..as long as it shifts ok and goes into gear right away it should be ok..unless it doesn't then you can have all kinds of fun with it..either way will be good..there is a lot of stuff out there for the 400..I have used in the past a chemical additive called Trans X..only need a little bit and it does wake the seals up inside..that's the last chance stuff that let's you know if the trans internal seals are getting weak..if it works I mean..don't use it unless it doesn't shift right or acts up..
Rich

Yeah...pour some of that stuff right in there...it's good for business....


Seriously tho...a lot of additives that you can buy to soften/swell seals do work. The problem is...the seals will continue to get softer, and swell bigger, until they fail completely. There's nothing in the chemical make-up of that stuff to tell it when to stop swelling the seals. It's definitely a last resort deal, imo, if you're trying to sell it, or just want it to stop pee-ing on your floor until you get time and $$$ to fix it. (no disrespect to Rich, or anyone else that uses stuff like that...)

There's no seals that hold fluid in the converter, so that's a bust. If the converter is draining back excessively, it's either the pump bushing(that the converter rides in), or a worn pressure regulator valve. Either of these things will allow air to enter the converter when at rest, and then the fluid just basically flows right out of the converter, right back into the pan(think of a straw full of water, with your finger over one end. When you remove your finger, the water runs out...same principle). This can also force leaks in places that don't normally leak, due to the high fluid level. A "full" 400 will have the fluid exactly at the height of the pan rail...right where the pan bolts up to the case.
Too high a fluid level CAN cause a situation where the pump can create an air bubble, and pump it backwards into the pan...that will spit fluid out the filling tube. But....it will also spit fluid out of the vent tube first...unless the vent is stopped up...
Adams' Apple2010-09-22 19:50:51

That's what I meant..last resort..sell it and let the next guy fix it!!  Definetely job security!!!!  don't use it if it shifts right and goes into gear normal..easy diag for people that don't have pressure gauges!!

My '80 Vette has 28K miles.  I have a transmission fluid leak that I can't seem to fix.  I have replaced the pan gasket and replaced the front seal but it still leaks.  The leak is about where the tranny and oil pan meet and a littlle towards the driver's side.  Any ideas?  Thanks.                                    Chuck

I have seen them leak where the shifter shaft goes in the side of the tranny. If it's not the pan gasket that may be where it is leaking. 

I'll check that.  Thanks.