My father and I rebuilt my 4l65e at his repair shop and one of the upgrades we went with was the larger billet sonnax servo. We got everything in and together, normal driving is fine and everything works great. But as soon as you accelerate hard enough for it to downshift and pass someone (not necessarily wide open) it will downshift and still pull. But when you accelerate from first gear again it shifts into 2nd really really slow and will not shift into 4th. After it did this we took the servo back out and it was stuck pretty tight into the case. We were able to use a pry bar to get it popped out. We examined the servo and put it back in, but it ended up doing the same thing again.

Has anybody had or heard of a similar problem and know a fix for it? Please let me know, thanks guys!

 

What's your band clearance, After you assemble it how far can you push the outer cover in till it stops? Shoot some air in the port on the pan gasket rail to air check for leaks and get the cover all the way out before you push in to measure. . I had a billiet od servo do this once, fix was taking the small return spring out. It was binding causing no fourth. What exact servos do you have? Is there double o rings on the intermediate piston? Chances are it wasn't set correctly and the band is burnt already.