Re: TH400 Won't stay in park
Before doing anything inside the trans, I would recommend removing the manual linkage from the transmission.
Turn the shaft fully clockwise to engage park. If the trans will engage park the problem is a linkage adjustment. I see that you already have had the linkage adjusted, but to be safe this test will cost you nothing but a little time.
The parking linkage is very robust and not likely broken. Since you say it never worked after the rebuild, I will assume it was installed correctly at the rebuild and the adjustment of the linkage is at fault.
My other car...Can beat you, too!
1967 Skylarks: (Swampthing)(Banana Hammock - 12.77@112)(Boat Anchor - Race Car - firstname.lastname@example.org)(GS 400)(Blue post)(White car - 12.42@115)(yes, I only own 6)
1971 Skylark custom convertible
2004 GMC Duramax (Towtruck), 1986 Regal, 2010 Camaro, 2002 Pontiac GTP
1987 GMC (POS), 3 MC, 1 lawnmower, 1 snowblower, 3 cats(Houdini, Trinnie, Isis)