Break Problem need help

My car: 69 Cadillac de Ville

Problem: With the normal (slow) footstep on the brake the pedal to soil and it nearly do not sink happened (hardly braking action) pumps bring also nothing. The brakes have no air in system. Times it comes times goes it.

With the a fast footstep on the brake the pedal remains hard and I has the braking action as before the problem.

Which construction unit concerns it? Did someone have already the same problem?

Please help me.

greet from Germany

Have you checked the brake master cylinder? The master cylinder will have a vacuum connection. Check it to make sure it's connected and that it's not leaking. Also, it sounds like the master cylinder could have a worn hydraulic seal or the some other worn internal part. If the vacuum connection is fine, and if the brake calipers are working correctly, I would begin to think that the master cylinder may be worn out. You said that there is no air in the system. Does that mean the master cylinder does not have any air. If you have not bled the master cylinder, check for a bleed valve and bleed it.

Greetings from New Jersey, USA