That's not how the check valve works. The check valve allows flow from the booster to the engine. If there's fluid leaking into the booster it'll get into the engine even with a perfectly functional check valve.
The check valve is there so if the engine shuts off or you've got low engine vacuum you still have vacuum stored in the booster to enable power brakes.
you maybe correct come to think of it... i forgotten, its been many years since i removed my boosters and welded that check valve shut! thanks