Only enough for one good stop. 73 Duster

I seem to have a vacuum problem somewhere involving the brake booster.
We bled the brakes and they feel fine and stop the car fine. Actually, they work in general unless you pump them hard and then try it again immediately afterward. Not that there's much ice around Northern California but right is right and these brakes aren't.

I changed the check valve and the hose is good.

There do seem to be a few drops of fluid leaking from the front of the master cylinder when the engine has just been shut off. Certainly not much, though.

Any ideas?

Thanks!