From
www.chevytalk.org The problem with the tranny slipping on turns, even though the fluid level is OK is a very simple, and cheap to fix, situation. Unfortunately most people don't know how to fix the problem, and it goes on so long that it wears out the clutch discs and causes other problems.
The initial slippage problems on turns is caused by a 50 cent O-ring located at the top of the pick-up arm/filter. This is accessed by taking the cover off the passenger side of the tranny, right where the filler tube is located. After that cover is off, you will see the pick-up/filter assembly. Remove the two bolts and you can slide out that assembly. On the end of that assembly is a flat O-ring that is causing air to leak into the system when you make a turn. When going straight, the fluid level is above that portion of the pick-up tube so it will just suck in fluid instead of air.
With any luck, you can replace this seal, replace the side cover gasket, adjust the two bands, and you will be good to go. If not, you may have to change the clutch discs in which case you should do a complete rebuild. I would not go to the guy that you talked with before since it is obvious that you would be paying for his education of Powerglide operation.
Once you drain the tranny, I believe that you have six 1/2" bolts to remove to get the side cover out of the way, including the filler tube. Then I believe there are either two 1/2" or two 7/16" bolts inside to remove that will release the suction tube/filter. They are right at the front, upper right side. With it will come the rubber sealing ring. Once you replace it, and install a new side cover gasket, it is a good idea to replace the o-ring at the base of the filler tube. The filter itself is a very fine brass screen and you can clean it while it is out. Put it all back together, fill the tranny and you are good to go. If you need to adjust the low and reverse bands, again, there are no special tools needed. In the service manual are the instructions, which are pretty straight forward.
The condition is usually pretty common in vehicles that are not driven on a regular basis, or sat idle for a period of time during their life. While sitting, I guess the seal is exposed to the air and hardens/dries out somewhat. Cheap fix to an otherwise good tranny, and can be done without dropping the tranny. I'm sure there have been a lot of Powerglides that went for a complete rebuild because they didn't know what was causing the slippage. Obviously after a complete rebuild, it would be fixed, but at a much higher cost.
Let us know what you find. The seal in question is clamped in there fairly snug. I have never seen one leak unless it was put in wrong. I would suggest a pressure check, at idle, on the low band apply port. There is a small steel flap valve that keeps the front pump from pushing oil into the rear pump pressure side and vice versa. If that is broken, it could also be a cause.... A leaking seal causes it to slip when you make a right turn, not hop out of gear while sitting still. As I mentioned, the slipping condition may have gone long enough to cause, or aggravate, other problems.
From Inliners International - Gordy 250
I don't recall a baffle in the sump. The only one I ever worked on with similar problems had a crack in the oil pickup tube near to where it attached. Apparently when cornering the oil would pull to one side enough to expose the crack allowing the pump to pull air instead of oil letting the band or clutch pack disengage. However, that transmission did not temporarily disengage at stops.
From Jalopy Journal
Try adding extra fluid in it even though it marks full. He said that if did not work, remove the cover on the passenger side were the dip stick goes, after that there should be two bolts that bolt a screen in place. Check the screen to see if it is partially clogged or torn. Also see if it has its correct o-ring or grommet on it.
I don't remember any baffles and the aluminum case units also had cavitation issues on a right turn. Deep pans help, as does fuel cell foam cut to clear filter and fill pan but overfilling causes foaming and poor clutch/band applications and burn up. The stoplight issue could be valve body half/gasket misalignment or throttle valve issues.
