It could be mechanical. Maybe a separator plate gasket tore some place and is bleeding off fluid. Also, piston seals don't last forever, especially if they've ever been run with increased line pressure. Those can rip and blow out a small section but still function. It would feel much as you've described - a sudden slight decrease in shift firmness from that point forward.

4L80-E's are notorious for cramming checkballs into the separator plate so hard they round out the hole and get stuck in the plate. That would very slightly soften the upshift but totally ruind the downshift making it take a few seconds. I don't think it's a checkball problem.

I would drop the valvebody and have a look. Get a spare gasket set to have on-hand. You can always make the feed hole a little larger to compensate while the plate is out.

Good luck. Look at it this way though, if it does break you've got someone here who will give you his cellphone number to assist with instructions and pictures through every step of the rebuild.

 

Thanks James, after your post and talking with Vince from FLT earlier today I believe it to be a mechanical problem. Worst case, my pan is covered with friction material, next case scenario, wear parts that you've mentioned and a few others may be the culprit. The plan is to drop the pan this weekend and make a parts assessment from there. Fortunetly for me FLT has good stock on alot of these incidentals.

If she needs a rebuild I just may take you up on that offer

I will also send my tranny cal out to a few trusted tuners for there evaluation just to ensure it is good to go, however at this point, the tranny cal is what I'm most confident in.