What causes about a 3 second delay when putting my automatic trans. into gear? (reverse or drive). I put the truck in reverse and a few seconds later it "pops" backward into reverse. Nothing violent just delayed.

 

You don't mention what kind, year, mileage so I will give you generic answer.

Low fluid level.
Engine idle too slow.
Worn, hard internal seals.

 

sorry, it's a C-4 in a 1972 F-100 with a 302

 

How is it once it's in reverse? Does it shudder at all when backing up a hill? Does it shudder in any other gears?
Make sure you run Type F fluid only in this transmission, as Mercon will kill it pretty quick.

 

Make sure your TVR is resting on its' stop, you have reccommended RPM at idle, your Manual shift Linkage is adjusted correctly & is reasonably free of play or slop. . . . If it all is right, and you have a delay, without chatter, or a slam shift on engagement, then
your Reverse Band or Servo my be too loose & need adjustment or reverse link could be bent or getting ready to let go.
FBp