Weak 1-2 shift

Hi,

My 4L60E has missed a 1-2 shift just a couple of times at WOT. It happened fast, so I'm not sure what went on exactly. It either changed 1-2-1-2 rapidly, or the change lasted so long or it went off gear or slipped for a brief time so that rpm limit was reached. Then it did manage to complete the shift. It's also possible, that I managed to let up the pedal a little bit without realizing it, before it upshifted.

I also noticed that the 1-2 shift is a bit softer over the whole rpm- and throttle position spectrum than it used to be. It doesn't matter to the shift firmness, if the selector is on D or OD. It didn't make a difference earlier either.

Other shifts feel good.

My first suspect is leaking accumulator, but I'm a real newbie in this thing... I'm sure there're a thousand other possibilities.
Would anyone more experienced have any advice?

Yeah, go after the 1-2 accumulator.  Take off the pan, and there's only 3 10MM bolts between you and removing that accumulator.  Be real careful not to rip the paper valvebody gaskets.  They're technically supposedly to be compressed only once and are quite brittle at this age, but if you're careful you can probably get away with one drop.   They are cheap to buy from almost any repair house though.

You've probably got a cracked yellow spring and a cocked/jammed piston.   The Sonnax pinless pistons are awesome replacements, and I think Sonnax also sells a replacement spring.