brake problems

I have a 1982 Corvette and I am having problems with the brakes. I have replaced the mastercylinder 2 times and all four calipers. The brake will be fine for a while then we just loose the pedal. I can not find any leaks I have also changed the front brake lines. If anyone has any ideas it would be helpful.

I worked on an 82 about a year ago. It would be fine for a while but on a long trip the pedal would get really low. He swapped masters a few times, gave up, then brought it to me. I bled all the calipers and got some air from the left rear.
I drove it and seemed to be ok. Since I couldn't take the car on a 50mile test drive, the customer opted to drive it. Same thing, low pedal, air in the left rear.
He changed the caliper, it was ok after that.

Open the bleeders one at a time and see where the air comes from. It could be a caliper that leaks during vacuum instead of when under pressure.

Hope this helps,
---TEX---