You must have an aftermarket gage since the 70 series
did not come with a boost gage. If so, I'd check all the
lines.

You can't pump up the intake because some air will leak past
the turbo up through the air filter. Also, the intercooler
has a small drain hole inthe bottom and that would keep it
from pressurizing (well, you can do it but it quickly leaks
down). So I think you have to trace any vacuum lines back
from the gagae and test them isolated from the rest of the
engine intake.