However, the Schrader Valve spring and the seal are designed to contain only the higher pressure freon inside the system. With the hi system pressure present, the freon will push outwards on the spring plunger and make the seal close.

When there is a vacuum instead of freon the atmosphere has a higher pressure than the system and will force the plunger in the opposite direction. The seal has nothing present to resist the higher atmospheric pressure. Atmosphere will enter the system which is the same as vacuum escaping.

The Schraeder valve concept only works in one direction, to contain higher pressure.