1999 Suburban engine problem

I have just swapped a new Vortec V-8 engine into my 1999 Chevrolet Suburban K1500 4*4, now when I start the truck and put it into reverse (foot still on brake pedal), I let off of the brake and the next time I step on the brake pedal the truck stalls. Does anyone have any ideas on what would be causing this? I have no error codes on the computer, my fuel pressure is good, I just replaced the fuel pump. I think that it might be a sensor somewhere, but I am not sure where to look.

Does it only do this in reverse? If you are in DRIVE will it stall also?

Brake booster? Perhaps the check valve?