.

drilling holes would not help you,in older t-stat housing it might,but
in 83 SD nope,you have to find where and why you loose pressure.

it may be simple as overflow jar cap (if you have one) or radiator cap.
did you try to squeeze upper radiator hose on cold and on warm?

did you burp system properly?

however testing t-stat before installation as described by diesel911
is highly recommended.

cheers

.

__________________
w126 500SEC gen II euro, powered by OM617 turbo stolen from 84 300SD 2.88 diff,EGR blinded
next wish/project: w114 coupe OM603 powered