Heavy Duty Check Unit The Parker heavy duty check units are essential in the use of industrial compressed air. These check units are part of the 'Schrader Original' range of heavy duty pneumatic check units from Parker Pneumatic. It also eliminates hose kinking due to it allowing the coupling to swivel. The twistlock design prevents any accidental disconnection. These check units have been designed to enable to user to connect or disconnect with a single hand.