Still chasing this problem. I'm at a loss here...

Fresh 518 rebuild with maybe 300 miles on it now. 1-2 shift is fine. 2-3 takes much longer. Actual shift is ok, just delayed. Transgo shift kit installed to RV/Truck spec, 3.8 KD lever. Even with the KD linkage off it's delayed, usually around 25-30 or sometimes even 35 mph at light throttle. When I assembled the trans everything was air tested and it sounded like the pistons were engaging correctly. All seals and rings were replaced.

What else could be causing this? Incorrect front band adjustment? Could the seals not be installed correctly? OD/LU solenoids leaking? Sticky valve in the VB?

I'd really prefer not to remove the entire unit to repair, but it gets annoying when it won't shift.
Makes it even harder to tune the fuel injection when it holds 2nd gear. Any diagnosis tips would be very helpful.