Im having an issue with my brakes and im not 100% sure what the issue is so i would like some more opinions on the matter. i have a 1990 corvette and the brakes are not working. The issue started with the car loosing brake pressure the more i drove. I would get air in the lines without losing any fluid out of the reservoir. This is what im confused about, i have seen online this can be caused by the seals on the pistons of the calipers to be bad because they pull in air when pressure is released but hold a seal when pressure is applied. I bled the brakes all the way around to find lots of air within the brakes. The brakes worked flawlessly after this but only for a drive or two before they were back to being very squishy with no braking power. Then the brakes got very stiff with no braking power. Just a few days ago i replaced the brake booster because it was leaking, the seals were bad and now the brake booster itself seems to be working properly (the brakes are no longer stiff, back to being very squishy with little to no stopping power). After i replaced the booster i bleed the master cylinder and the brakes and they were working great for a drive or two again. Now there is just no pressure when i push the pedal and i cant even get them to stop the car in neutral. could this be the calipers leaking and pulling in air but not leaking fluid? or could this be another underlying issue? would love any feedback anyone could give me on this issue, im just not sure exactly what it could be.

 

The air could be from calipers but don't remember that being an issue on c4s. C3s yes.
Usually no pedal pressure is the master cylinder.
Old flex lines collapse over time internally, but if they all bled out at the calipers that dosent sound like the issue.

 

How many miles on this car? Have the brakes ever been serviced? When you say you lose pressure, is the pedal going to the floor? If so, can you pump up the pedal, and will it hold, or slowly drop? Possible rotor run out, Has that been checked?