This brake system use power brake booster check for vacuum leaks frist if is ok check hydraulic system for leak. p.s you can let us do a brake inspection and diagnostic to see what is the problem for your vehicle. The brake inspection and diagnostic is $55 for more info pls call 650-992-1900 to make app with Andrew Thank You
What's the fluid look like? Spongy pedal usually means air (it will compress) so try and bleed the system. Start at the right rear, left rear, right front then left front. Use a high quality fluid from a new bottle.