Can't burnout:(

We took a vacuum gauge to the booster while running in gear. One guy had the vacuum cap off the other port on the check valve. At idel in "D" it shows around 17psi. I then loaded it up trying to do a burnout. The gauge then showed around 5psi. I don't think it should be that low. The cam is only a 340 spec so nothing radical. Possibly my booster is going bad.

I was thinking about finding a large empty parking lot, disconnecting the booster hose from the intake and plugging it. Then trying to burnout. Would this be okay just to try? Or would I cause other brake issues? I guess I would basically have manual brakes.

I have another question though. I have had people tell me to start in "1" then powerbrake it to burnout and others say to start in "2" So what difference does it make? I thought it would be okay to start in "D"

Thanks again for all the feedback. Info is still appreciated.
The drop in manifold vacuum with throttle application is normal. The harder you work the engine, the closer it moves to 0" hg. I wouldn't think the pedal should go stiff that quickly though.

My understanding of the potential hazard of starting a burnout in 1st is that if you back out of the throttle, the shock generated may scatter the sprag. I may be way off as I'm still getting my feet wet with how a TorqueFlite works.