The reed valve only opens shortly after start up and during deceleration. An easy test with a warm engine is to remove one end of the large hose. Rev the engine to ~3000 rpm and release the throttle quickly. You should hear a loud pulsing noise from the reed valve until the rpms drop to near idle speed.

If no noise or you hear it at idle, the valve is bad. It's subjected to combustion by-products so it degrades internally. Those I've disassembled are either clogged or very corroded inside.