You should see a multitude of tiny bubbles passing through the site glass, basically pin size bubbles. With it dropping when rpms are increased, you probably need to add some R134 if you have the equipment to do it. You might also have an A/C shop exchange out the R134 for some new.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed