First off, this 4L60E is in a swapped classic car. My combo is in a ls swapped vehicle and the engine and trans is installed with eng/trans tilted down a little lower than stock at the rear to prevent cutting the trans tunnel. It all started last season when I went to the track and manually shifted up the gears the 2-3 shift would slip and the engine would hit the limiter. So I raced the rest of the day leaving it in drive. This season, is was doing it while shifting and when in drive. So I thought the trans was on its way out. I installed a transgo HD-2 into it thinking maybe something was sticking and with the boost valve I could possibly raise volume/pressure and make it go away. I also installed the sonnax "check valve" 77701-076. Put it together and it was worse! now it was slipping at 1-2 and 2-3 only under full throttle.
Thanks to someones post here on Ls1tech that said to carefully over fill the trans oil level slightly my problem was solved! Apparently when you increase how your car launches the oil can flush back on these 4l60E's and loose line pressure. This was exaggerated with my trans slightly tilted downward. I also read that converting to the truck oil pan (because its slightly deeper) and slightly overfilling the trans level can stop this.
DON,T OVER-DO IT THOUGH! TOO MUCH WILL HIT THE SPINNING PARTS IN YOUR TRANSMISSION AND FOAM UP THE OIL AND BLOW IT UP.
I was searching this site and others when this started and saw many had transmissions rebuilt and replaced and spent tons of $$$ only to suffer from the same issues. I thought I was in for expensive repairs when all this time it was $5 bottle of trans fluid. Hope this solves your problems.

 

Thank you for the reminder that overfilling by 1/2 quart ATF often solves the 2-3 shift flare.
As you mentioned, install the truck pan also helps.
Strange that installing the HD2 kit made things worse; I suspect there was a problem with the boost valve.
What year is your 4L60E?
I recommend the Sonnax boost valve even when installing the HD2 kit.

 

This is especially true if you are using an aftermarket dipstick which may or may not read correctly.

Fairly certain it led to premature 3-4 clutch failure in mine before. It had started slipping again at WOT on 2-3 highway pull shifts, added 1 qt fluid and it went away.

Come to find out the Lokar dipstick was not correct as far as the "full" mark was concerned. I discovered it while I had the pan off to replace the filter in hopes of making the trans last a little longer...ever since filling it more it's been flawless.

 

This goes back to the most overlooked item when installing a trans, verifying your dipstick level is correct. Even if it's a stock, it needs to be checked. Keeping the fluid at the pan rail works well.

 

I have a Lokar dip stick in a 57 chevy LS3 4l65e hate it , what is a good after market dip stick ?

 

When I built my first 80E, I had a pressure gauge hooked up to it, and noticed it was still building pressure within a qt or 2 of full-the dipstick was a modified GM one, so the mark was accurate, verified with the pan off.
I wondered if 60 & 80E trans were built with the min. amounts of fluid, you know, weight saving, gas mileage, etc-a little here, a little there to get down to certain Gov. imposed #'s, lol
Good info, esp since you proved it

 

Maybe the hd-2 neded more fluid volume with bigger boost valve, tougher springs and enlarged valve body spacer plate drillings. Might have worked some of the air out of the trans too and the level dropped even more. My trans is a 2000 camaro with the 5.7L.

 

Big fan of overfilling the 4L60e, the one in my daily has 175,xxx miles on it and it still shifts like it was new. Its going to break my heart to blow it up with a turbo lol. I always thought that if you put to much fluid in it would just run out the vent tube, I never knew it could damage the trans. I've always ran mine at least an inch above the hash line when hot.