Hello I have a very fresh rebuilt 400. The problem is when i stand on the thing it slips in sencond gear. Ive changed the mogulator checked vacum. The weird part is that it will blow the tires off in first then when i shift to second it seems there is nothig there. If i let it shift it self And dont stand on it. It seems to shift but very lazy. If i let it shift it self and gas on it a little it seems to go into 3rd almost right then. Thanks for any help.  

Coudl be one of the check balls in the valve body is missing, a bad spring, or the bands aren't tight enough.  

hey guy, I'm not a trans man but I had 400 in 69 truck the same thing happed and it was the sprag ether real weak or striped. hoped this helps you.

Thanks
Lee  

I dont think it is the sprag because it worked fine till I had it gone thru. The only reason I had it gone thru is because it went from being behind a 355 to a 388 with N2O.  So I guess Ill have em pull it back apart. Just to be safe. Thats all I need is this thing blowing apart on me. Thanks for the input.  

I found the problem. It was one of the check balls in the valve body. It was missing. Thanks for the input.