2006 1500hd no reverse

[jestersht]

hi i have a 2006 gmc 1500hd with the 6.0l the tranny went on it so we replaced it with a brand new one that was installed by a dealer, well it works fine except when you go out in the morning and go to back out of the driveway it wont move but if i put it in park and let it warm up for 10 min and try again it works fine we have had it back at the dealer to have it looked at and they said its fine so i took it somewhere else and they said its fine.
so i am hoping someone here will know whats wrong with it becouse ive changed the tranny filter and fluid spent about a grand having it looked at and cant figure it out

[gmtech1]

Is it an Allison transmission? RPO code M74 or MW7? The codes are listed on the silver sticker in the glove box. If it is,take it back to the dealer as a bulliten was released just for this concern. Bulliten is #PIP4379J. It has to do with low or no pump pressure due to the pump losing its prime. Also, check and make sure the fluid level is ok.

[jestersht]

Thanks ill look into it, the thing i dont get is drive works fine.
Fluid is good as i just changed the filter and replaced fluid with new
but ill look into the dealer thing and im not sure what trans is in it but ill look later when i get back and it worked fine until the new trans got put in