77 vette brakes

Here is the brain stumper. New Master Cylinder, I rebuilt the calipers, new Proportioning valve, New rear brake hoses, new rear steel brake lines, brake system fully flushed. (Midas replaced the left rear caliper, I rebuilt the others,) I cannot get the air out of the system. I have Used a hand vaccum pump, and had more air in the system than when I started. SO I bled the system by pumping the brakes. Wht Happens, I can bleed the brakes and get a pedal, now, not a great pedal, it stays a little spongy, But about a half pedal. I drove t the car for about 3 hours through some backroads, and the longer i drove the worse the brakes got. I'm sure there is air back in the system. This has been an on going problem. Any body that has the answer,
PLEASE let me know. I Like the car But I'm getting dang tired of working on the brakes. I want to move on to other things, like restoring the inside.
:banghead:
Dave
Spotsylvania Va

This is a followup on my post. The calipers were getting air in but not leaking fluid. I found out that this was a problem with these Vettes. It was recomended that I replace the calipers with ones that had a stainless steel sleeve. I found them at Auto Zone at a reasonable Price. Now, I have great brakes!! I just wish I had found out sooner.:grinyes:

Dave
Spotsylvania Va.