brake problems?

I have a 1970 gto and I am having some brake problems, I have a new master cylinder that has been bech bleed before installation, and I believe it is working ok, but when I pump the brake pedal with the engine off the pedal does not get hard, and travels all the way down to the floor, with the car running, once the pedal is touched it drops to the floor and will not return until the vacuum supply from the engine is disconnected. I have had the vacuum booster restored and was thinking it might have been put together incorrectly and casuing this condition. All the lines and calipers are new, but the wheel cylinders are a couple of years old, but not leaking. I have also just installed a used proportioning valve. The metering valve is original, but I can't see it causing this condition. If there are any ideas please respond. thanks matt

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