I have a 1994 Del Sol and the brake pedal has alot of travel and is spongy. With the car turned off the pedal feels normal. So far I've replaced the following- master cylinder, booster, proportioning valve, and both rear calipers. Pads and rotors are good. Brakes have been bled properly and all fluid is new as well. I'm out of ideas

 

Caliper slide pins stuck maybe? You likely still have air in the lines. Bleed the fluid with the car turned on and use a 1 man bleeder. The pump and hold method is extremely inefficient.

The pedal gets firm after you turn the car off because the booster is not active after a few pushes.