1988 Chevy, 700r4 Wont hold in first gear.
I recently changed the transmission fluid, filter and gasket on my 88 Chevy, 700r4. As far as I know the only mods are a shift kit. Now if I shift manually into 1st and go, it will shift into 2nd like normal at about 2000 rpms. It does this regardless if I'm driving hard or not. The fluid level is good. It shifts fine automatically when I'm in D. I did some experimenting.....I drove it in 1st gear and kept the rpm's right about the shift point. It seemed as if it was slipping between the 1st and second. I used Valvoline Dextron VI ATF full synthetic when I did the fluid change. It states that its good for all GM tranny's old and new using Dextron tranny fluid. The fluid level is good. Should I be looking into this fluid or elsewhere? Additives? Thanks.