Brakes on my 87 El Camino

[enio45]

I have an original low mileage 87 el camino - 20K miles.

pads, rotors, and all the brake system is orig to the car. I feel like the brakes are spongey - like if i wanted to, I cannot LOCK them up if needed. It will stop, but to a slow roll, not lock up.

I bled the system and flushed out the fluid and installed new fluid. No real change - so im trying to figure out where the issue may be - booster, master, calipers and rear wheel cylinders need cleaned out and redone. Not sure best way to separate the diagnostics to pin point he issue, or am I at a trail and error?

Any advise? I have no leaks in the system, just a soft feel to the brake pressure.

thanks in advance.