Pressure Test can show leaks anywhere but not WP function.
Problem is most test sets won't fit Fiero because rad cap area is too small to fit the adapters w/o hood removed.
You can try drilling out Tstat cap rivet and replace w/ bolt in tire valve assemble.
Can Pressurize the system until rad cap think the car is at "boils over" but likely leak will show up before that.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave