first, third has no clutch or piston seal, it is direct drive through the transmission, all this does is verify the band is engaging. this also has nothing to do with the valve body, but the reverse does. the fact that you have have reused a torque converter just leads me to believe me to believe i am that much more correct, especially if you had it installed in a tranny that went bad.
you need another transmission, or at least you need to replace the clutch drums and sun shell. pressure loss explains your problem exactly and even though you may not like it, you have a bad tranny.; time for a new one, or at least a known good one from a wrecking yard or somewhere else.
you would probably not get a code from a weak plug in the side, and its definitely worth a try, but im 99% sure that is not the case, as if it was you wouldnt be able to start the truck.
the 3-4 clutch as it is called, is 4 th only. its the transition from 3 to 4 that makes it a dual style clutch,
i am currently dealing with the exact problem you are having, odd as that sounds, but there is one thing different, you have reused an old torque converter from an old transmission. right there i would throw out the entire tranny, and i can contribute all the problems you have to that alone. my problem is from a bad case, just have to find where.
the problem will not be in the computer but in the tranny, ill opt pout so some1 else an answer, but after specializing in GM transmissions for the last 6 years, you have a bad case or piston seal.
Chevy Technician
schooled, trained,love the wierd stuff