I have a 1966 coronet with a poly 318 and a 727 transmission, from the first day I bought the car when you start it up in park it seems like its partially in gear cause it lurches forward slightly but does not roll forward, it does the same thing in neutral if you rev the engine it seems like its trying to move forward but doesnt (if that makes sense) the car had the kickdown linkage on the car but wasnt hooked up so I installed it and adjusted it and the trans never slipped but seemed to shift real early no matter how I adjusted the linkage. When I restored the car I sent the trans to a friend to be rebuilt, at the time I never messed with rebuilding transmissions because he would do them for me and my friends for a good price and never had any problems. He looked it over and said he did not see anything wrong out of the ordinary except the clutches were worn and the frictions were warped from heat, when I asked him about what the car was doing he said thats probably why its moving forward slightly cause the frictions being warped it was aplying pressure to the clutches enought to make the car move. I bought a standard rebuild kit, a trans go shift kit (for a nice firm shift) and a new/rebuilt stock convertor. After getting the car back together, It does the same thing as it did before. Me being the lazy guy I am and trying to (raise young twin boys) I went withe the old saying well it runs and drives so I have been enjoying the car for the last few years, but would like to get it sorted out. I have looked all over the internet but have been unable to find much info on the same kind of symtoms, Its seems to me like maybe the valvebody has something wrong like there is an internal leak that applies some pressure to the clutches to make it lurch forward in nuetral or park but wont allow the car to move unless in 1,2, D or reverse, and also have a lazy/ soft shift. The car has been driven probably at least 100 to 500 miles per year for the last 5 yrs and the trans has not shown any signs of getting worse. Any help would be greatly appreciated.

Lurching forward in Park and Neutral is a known problem with the pre-'67 transmissions, this is caused by trapped fluid in the rear clutch retainer, the trapped fluid causes drag even with the released clutches. In '67 they started drilling holes in the drum periphery to allow the trapped fluid to escape.

Thats cool! Learned something new. Thanks John.

Thanks for the reply John, you are a wealth of knowledge, I would have never thought it would have been something that easy, I would of been bummed if I tore it apart and put it back together only to have it do the exact same thing. I guess If I have time over the summer I might pop that out and either get one with holes or take it in to work and add some myself. Any advice on the early shifts other than adjusting the kickdown linkage?