The Combi/prop Valve and MC can get Crushed ports stopping flow to calipers.
Is cause by fools that over tighten the steel nuts and lines into the aluminium bodies.
Can See some of this damage in my Cave,
Combination Valve where some ports are nearly crush close where the steel flare end hits.
Otherwise Combi Valves rarely go bad. Can get crap in them blocking fluid flow or cause binding of moving parts.
Only GM Part made Combi Valve available via Dealer Parts but stopped making/carrying most Fiero parts Decades ago.
Getting used combi and some other brake parts is iffy at best because often means this part was stored out of a "sealed" system and brake fluid sucks water out of air then "rots" the guts.
Spongy Pedal is not a Combi problem.
Most often you have air in the lines, calipers or MC.
MC often needs "Bench Bleeding" if replace or just tank runs dry doing other work... See my Cave,
Bleeding MC notesMight get MC air out if you jack rear of car to level out the MC or tip MC front a bit down. But you need to jack the rear of car high and that can be dangerous in several ways...
Low Pedal is not that problem too.
Low Pedal in Fiero is often cause by rear calipers with iffy pistons that won't self adjust. See my Cave,
Rear Brakes &
Rear Piston notesI list spongy and low because can have both problems or confuse the two terms and isn't = to say.
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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.
(Jurassic Park)
The Ogre's Fiero Cave