Ok this isnt a chevelle but hey brakes are the same on most.
1950 chevy 3600 Truck
Brakes are non-power single piston m/c
Rebuilt Wheel Cylinders
New M/C
New Brake lines hard and rubber

Problem is the pedal will go to the floor but if you pump it a few times its firm and holds until you let off then in about 5 seconds and you press the pedal agian its all the way to the floor and you can pump it up and its firm. Would this be a faulty master cylinder?

HELP Need to get this going by the weekend.
THANKS!

Would this be a faulty master cylinder?

HELP Need to get this going by the weekend.
THANKS!
If you've blead it a few times and are sure there's no air in the system anywhere ? that's what I'd check next.

I've experienced the very condition you describe on a couple of different master cylinders. Both times it was a leaky internal seal and I had to either rebuild or replace it.