tranny troubles

Im having trouble with my c6 tranny. First it wasn't shifting so we replaced filter and modulator valve. Now we get first and second but no drive. Its hard to shift and really has to rap out before it shifts from first to second and won't shift to third at all. can manually shift from first to second with no problem but when you go to drive it shifts into neutral for about five seconds then goes into second. Could this be because of wrong or bad modulator valve or is tranny shot and needs to be replaced. We got engine and tranny out of a ford van and when we pulled it everything worked fine. All help appreciated.

check the tv (throttle valve) and cable sounds like its stuck on wide open

The C-6 does not use a TV cable, they are vacuum modulated.

I've seen this a dozen times, every time the vacuum line to the modulator is either off/broken or plugged so the trans *thinks* you are a full throttle all the time, so it upshifts late (1-2) and never (2-3).

Trace out the vacuum line to the modulator and make SURE it is good and connected.

tried changing out line with vacuum hose and now engine will shift manually up to second and when you put it up to drive it still doesn't shift to third but doesn't go into nuetral. Could the place where vacuum is coming from be the problem. Have it located at the back of the intake manifold. Also this problem didn't start happening until after the carb was rebuilt.