An air compressor check valve has only one function: to let air flow from one side to the other, while blocking air flow in the opposite direction. If you hold the check valve in your hand and would blow on it on one side, you have no problem blowing air through it. But, if you turn the check valve around, it is impossible to blow any air through it. The same thing happens inside the compressor or compressed air system. Once the compressed air passes the check valve, there is no way it can go back again.
CAST BRASS HORIZONTAL INLINE CHECK VALVES