It sounds like you have a kink or a leak. I had the same issue as you with my Sterwart Warner boost gauge. Found the leak/kink (was both! ) and all was fine.

BTW you can build boost in "Park." At least mine does!

 

i have replaced the lines and redid the ground and i still get a -20 vac (idle, driving).
the most it boost at is like +5 if that.

anyone have any ideas what is wroung? or am i not reading this right?

 

I had an Autometer boost gauge for a while and it worked great.

Have to wonder if the gauge is defective itself ...
You've done everything else already.