My wife got into her 2017 Suburban 5.3 Premier started it and when she put the gearshift in reverse it started rolling backwards but she said it seemed quiet and weird. She stepped on the brake but no response. She applied the brakes repeatedly but no braking action. She then quickly put the gearshift into park. She said the vehicle jerked to a sudden stop.
She tried again and same results.

She called me and I came to where she was. I got into the car and everything worked fine. Could it be that the motor was not running and that would explain the sluggish or no braking? Thanks

 

no it should still stop. Test it yourself with the engine off.
Was it a really steep hill? was she wearing high heels?

 

No just a slight sloping driveway. She was not wearing high heels. She said the break pedal was very hard…..would not compress. Thanks for your quick response

 

ok maybe the engine stalled on her (something you may want to be on the alert for).
without power assist it requires more force on the pedal (but not an unreasonable amount) to hold the vehicle, and maybe she just wasn't prepared for that. (I'm sure this is described in the owner's manual)

 

+1 for engine off. you will have 2 presses of the pedal until the vacuum assist is depleted and then the pedal with be hard. This should be part of driver training so people know what and what not to do when the engine dies at hwy speeds.