The problem usually only manifests at first start and idle. Once the car is moving and the engine starts pulling a vacuum,mine works fine. And so long as the check-valve is working there have been no issues.( 292/509Mopar cam). Is your idle timing perhaps lazy?
Timing is fine if not advanced just a bit for the 1/4 mile. But you described my symptoms. Once the car is moving and the engine is at operating temperature the brakes are fine.
