Most likely the 3-4 clutches are on their way out.
If you look at a range reference chart for the 4L60/4L60E you can get an idea of what’s happening. To get 1st gear, the forward clutch is applied, the low roller clutch is effective (and remains effective or “holding” in every forward gear) to shift to 2nd, the 2-4 band is applied. As the name suggests, the 2-4 band is used in both 2 and 4th. To shift to 3rd gear, the band releases, and the 3-4 clutch is applied. Once again, the 3-4 clutch is applied in 3rd and 4th. To shift to 4th, the 3-4 clutch remains applied, and the 2-4 band is applied.
The reason it won’t slip (YET!!!) in 4th is due to the gear ratio. The torque load the clutch sees in 4th gear is much lower than in 3rd. Eventually, it will slip in 4 gear as well.
The 3-4 clutch has been a issue with the 700 since day one. Have the shop that rebuilds it install the latest design 3-4 clutch kit. HydraMatic originally designed the trans with 5 clutches. Then as engines became more powerful, they added a 6th clutch. With the 4L65/70E trans, they added a 7th clutch.
The problem with just added extra clutches is fitting them in the same space. To fit 7clutches where 5 fit required some dimensional changes to the input clutch housing, in addition to making the clutches and steels thinner. Thinner frictions/steels don’t handle the heat nearly as well, get them hot and they warp. It’s all downhill after that.
Raybestos offers the Zpac, it’s basically 12 single sides clutches and steels. That fits in the factory housing with the proper common GM parts. The Zpac is a more expensive upgrade, unless you plan to really beat on the truck the factory 6-7 clutch setup is probably good enough.
While you won’t have any clue, hopefully the shop reinstalls or replaces if worn the load release springs. Because of the lousy clutch life, people were tossing the springs in the trash,thinking they caused the clutch to slip. The springs are actually designed to keep the clutches from slightly dragging when released. GM has felt they were needed, who knows how many millions of those transmissions they have built. If those springs only cost a penny per transmission, that still adds up to a huge amount of money. If they weren’t needed, GM wouldn’t put them on there.
