My 90 was/is developing brake problems also. I experienced somewhat of a "lock up", actually more of a "drag" on the rears. I'm guessing you are sure it's all four and not the rears?
No insults implied or intended. The helm has a section for brake trouble shooting, it's a "look up" chart in the brake section. Humor me for a bit(I'm old & senile
). If you have a FSM look at the chart. On mine when the rears were "draging" the chart said do the parking brake & cable adjustment and brake pedal switch adjustment.
Brake pedal switch adj = pull up on the pedal & listen for a rachet sound...if there is a sound, pull up again & repeat gentle pull up till sound is no longer.
Parking brake & cable adj = pull up lever & release at least three times. When finished the lever should move up 3 to 5 notches w/61 lbs of force applied to it. That's it on the simple stuff, all of 10 mins.
When the pedal feels like a rock, do you get what you would consider acceptable stopping results? If not the symptom is pointing to a booster failure, maybe. The booster is unique to a Z, dual diaphram & NLA from GM. The repair kit is also NLA. Cardone & a couple of others make reman'ed units with a core exchange. If the master leaked fluid out the back seal for a long enough time you will most likely ruin an otherwise good booster. Fluid disolves the diaphrams as I've been told.
My car sat more than it was used before I bought(big mistake on my part). When I started using it and when I bleed clean fluid thru the system my master leaked into the booster. Look up at the top of the brake pedal & see if the push rod boot is wet. You will have to r&r the hush panel/knee bolster to really see the tops of the pedals. While you're ther look at the clutch push rod also & take note of any fluid. If ya have fluid on both plan on replacing both master cylinders....GM lists rebuild kits but the clutch one is BO'ed. Rock auto has the rebuild kit for the brake master....about $90 I think.
You can put a mity vac on the booster, suck about 10 to 13" & see if it holds. If it does then you're good....unless the master did leak. If it did it's only a matter of time.
One other thing....long shot unless you know for sure that someone didn't r&r'ed the booster + master...there is an adjustment on the push rod that comes out of the booster & hits the piston on the master. If it's too long it will close the "return" circuit in the master & keep hyd pres in the system causing a drag or lock up. Go to Classic Performance Products.com they have a Q & A section that I found useful...but I'm a novice so any info is useful to me!
Tom