Been working on the notorious 2-3 upshift flare (1-2 is great) on the C6 that is in my 1986 F-150 (351W HO, 4WD, reg cab, short bed). So far this is what I have done to it:
1) Replaced transmission oil twice with quality filters and ATF oil (first time I changed it, the oil was black but did not smell burnt, second time nice clean red with very little metal shavings on pan magnet)
2) Changed vacuum modulators from a purple stripe to a black one (when vacuum line was removed there was oil in the line so I knew it was faulty)
3) Removed valve body and cleaned it with petroleum based solvent as my C6 manual instructed
4) Made sure I was getting good manifold vacuum through gauge and double checked metal line and hoses from vacuum tree to the transmission

I recently saw a posting on a vintage Mustang forum talking about servos and levers. Just checked mine this morning, and I have the "N" servo housing. Through the info on the Mustang site, the "N" servo is the most soft one there is.
My question is, can I put another sized servo such as a D or P in a N housing? If not, I will just track down another N servo and go from there. The servo piston is the last outside part that can be removed before full rebuild, and I just thought it would be worth the shot. It kinda makes sense at least to try it because if the seals on the servo are bad that would be a pretty profound vacuum leak not allowing the lever to hold the intermediate band long or short enough for the gear change to happen. Plus I think that could possibly help my crappy shift timing the transmission has too (by 18-20MPH I'm already in 3rd gear). That might be normal, but in my opinion that's ridiculous.

To my knowledge, this the "original" transmission that came in the truck. I do not know the actual mileage on the truck due to having a 5 digit odometer as ya'll know. Worst case scenario and I am believing this more these days trying to fix this, is that the transmission will need to be pulled and rebuilt due to just sheer age and unknown abuse. If that is the case, I will be using a wide ratio gear set and a Hughes Fuel Miser TC.

 

The guts of the trans work on tranny oil, not vacuum do they? The only vacuum to the trans is to the modulator, which has a diaphragm in it and converts the vacuum to mechanical movement of a rod. The only thing I have ever seen help a trans on it's last leg was Lucas tranny fix. If it's something minor, this stuff will help it out.

https://www.walmart.com/ip/Lucas-Oil...24-oz/16777815

 

 

Also, if you want to nurse it along till you get time to pull it, lift on the gas pedal a little bit when you know the flare up is coming. That flare up is the clutch inside slipping, and if there is any way you can ease up on that it will help it last longer.