1976 Chevy Corvette Brake Bleeding

You may have air in the master cylinder. During the normal bleeding process, if you accidentally allowed the fluid in the master cylinder to dip to low, air could've been introduced into the system.

If you haven't already done so, disconnect the lines exiting the master cylinder, connect rubber hoses to the outlet ports and bend them up so they are submerged in the master cylinder. (Make sure it's full)

Have someone pump the brakes while you observe the fluid cycling through the hoses until there's no air bubbles present.

Then reconnect the brake lines and perform another bleeding procedure, starting with the left-rear, right-rear, left-front, then right-front.

If this doesn't solve your spongy pedal feel, then you probably have cracked/ruptured seals inside of the master cylinder, and it will require a rebuild.

Hope this helps.