I need help with the 400 turbo tranny in my 1976 Chevy 3/4 ton. It won't shift into 3rd gear. I replaced the vaccume modulater on the back of the tranny, nothing happened. The tranny fluid disappears. I find no puddle of fluid on the ground, but I keep pouring it in and it keeps leaving.

I live in a small town in the Ozarks and this is the only running vehical I have now. Any help would be greatly appreciated.

P.S. The only complicated thing I've done to the engine is replace the water pump. And that won't help me with this.