I recently purchased a '72 Ventura II which has a Pontiac 400 big block with THM350. I'm not certain the year of the 400, but I believe it to be out of a mid-70's firebird or ?? The block numbering was inconclusive. Anyway, when cold, the tranny will not shift out of first gear until the engine reaches 200+ degrees in temp. I have replaced the vacuum line, the tranny oil and filter, etc to no avail. I have moved the vacuum line from point to point as well, with no effect (it is connected to manifold). After the initial warm up, the tranny shifts fine with no slippage, etc. My reading indicates that maybe this motor had a vacuum delay, which does not permit adequate vacuum, and that it maybe should have an overide valve?? If that is the case, I don't know where to install the overide. I have been told to make a vacuum canister to create adequate vacuum?? I'm lost... any ideas please? Thanks.

 

You may have a sticking modulator valve. I don't see how that it would be engine vacuum related.